It seemed like Robbins and O'Neil were moving Batman in a more mature/darker direction prior to DKR (not sure what happened in the interim), although maybe not to the same degree. I haven't read nearly as much Daredevil and haven't read his run (I don't care for DKR), but what little I read would seem to indicate he probably had a bigger impact on Daredevil just because there's a bigger difference in the pre and post-Miller Daredevil I've read than Batman.

Miller transformed Daredevil from a second-tier, almost cancelled Marvel title to one of the early 80s more popular titles. His Born again run from #226 to 233 is not to be forgotten either.

So yeah, his impact on horn-head was bigger.

As I'm not as familiar with Daredevil as I am Batman I can't really compare the two, but I do know that Mark Waid is currently writing a version of Matt Murdock that doesn't seem to be too difficult to reconcile with the pre-Born Again version of the character.  I don't know however, if the same approach would even be attempted nowadays with Batman.  Even when doing a story inspired by and set during some of Batman's more outrageous Silver Age adventures, Grant Morrison still seemed to be using Frank Miller's (or at least, a Frank Miller influenced) template for the character.  I mean, if you can't revisit the Jack Schiff era of Batman without using Miller's unbalanced and obsessed intrepetation of Batman as your go-to setting, then yeah, it's hard for me to believe that he had an even greater impact on another character. 

Interesting fact - Frank Miller's involvement with Batman (to the extent that his ideas were severely changing the character) actually goes back to Batman and the Outsiders.  At the end of the first issue, Mike Barr gives thanks to a few other creators mentioning that they helped shape the Batman we'd be seeing in that title.  One of those creators was Frank Miller.  I don't know who came up with what, but the Batman in that series (at least with regards to the early issues)  really feels like the Batman we wouldn't really be introduced to until Dark Knight/Year One.  The team even finds its origins in Superman acting like a government stooge (something the character would never be prior to this story) and has Batman complimenting one of his men when he throws a dictator to his people to be torn apart ("I like the way you think" he remarks). 

That's kind of a tangent, but I thought it worth throwing into the mix.

Although I'm not a Frank Miller fan, he did come up with some ideas that were inspired. He was the one who figured out which movie the Waynes watched the night of their murder for instance, and by establishing that Alfred served as the Waynes butler since Bruce Wayne was a boy, he pretty much ensured that from this moment on, Alfred would play a far greater role in Batman's life than he had before. I can't help but be impressed by the way this minor detail now made it almost essential for Alfred to possess a dry, caustic wit that had been absent during his first 43 years of existence. A butler who only met Bruce Wayne after his career as Batman had begun would be out of line to sarcastically comment on his crimefighting, but as a member of the family who had helped raise him, it would never feel inappropriate.

I think however, that while Miller's impact on Batman was and remains unquestionably huge, it has nonetheless been an negative one. It strikes me as odd to hear about how much damage the Adam West series has had on Batman when the existence of that show never stood in the way of us getting the Robbins/Novick/O Neil/Adams version just one year after Batmania took off, or the Frank Miller version 20 years later, yet here we are almost 30 years after Dark Knight and it's the mentally unhinged, emotionally unstable, man-child that prevents any better interpretations from coming to the fore. Again though, I'm not familiar enough with Daredevil to say whether or not he has these problems, but it does at least feel as if a writer has more possibilities when writing him than he or she would with Batman.

I love Miller's work on both, though I prefer Dark Knight Returns over just about all of that. Dark Knight Returns was something I didn't take to right away, as with most of Miller's work, and only really took to when I removed the "realism" tag.

Having said that, he seems to have had more of an impact on the characterization of Batman (where most writers afterward, I would say, took the wrong thing from his work) and the plotting of Daredevil (DD getting his life torn apart is his "thing" now). I'm fine with the latter — superhero ongoings are cyclical — but am annoyed at the former, as Batman hasn't been a character I've enjoyed reading for quite some time now.

I haven't read his Daredevil work, but I never liked anything he did on Batman.

I also esteem Miller's Daredevil work much more than do I his Batman work.

I still clearly recall picking up Daredevil as it his the stands each months during those years of the early 80's.

It was both a transformation experience in my relationship with the medium, and, as it turned out would be last hurrah with comics for a couple decades.

As Miller's long saga unfolded month after month it felt to me as if it was the carefully plotted and deliberate telling of a novel, not the sort of scattered but ad hoc narrative structure that hitherto characterized much of comic storytelling (obviously, there were significant exceptions)

All told Daredevil was richly layered drama immersed not so much in the Marvel universe but in its own lexicon, interests and direction. It avoided sentimentality but neither did it venture into the depressingly; it felt authentic and organic.

By contrast, initially excited about Miller's Dark Knight (I was and remain a proponent of his Ronin mini-series), I actually never ended up finishing it, giving up on both the story and comics themselves after two issues.

I'm sure other external factors were involved, I was at a time when my drift from comics was inevitable, but nonetheless, it then felt to me that there was something a little gratuitous and even alianting in DKR. At least it's appeal never reached me at the time. I've since gone back and read the whole thing and my opinion has softened somewhat, but I still will without a doubt advance the vitures of Miller's Daredevil over his Batman.

Yeah, let's keep in mind that Daredevil was already one of the darkest Marvel titles when Miller came on board - Those Copperhead and Death Stalker story-lines were horror tinged and BLEAK, compared to the bright-primary-colors of the other Marvel books. And also, due to the Black Widow guest starring, the most frankly sexual book in the Marvel stable as well. I'd honestly say that Batman changed MORE due to Miller's influence.

Also Dark Knight Returns was insanely well put together on a formalist/technical level, and synthesized a lot of stuff that had never really been part of American Mainstream comics before, drawing from Greek Theater (complete with the media acting as "chorus") and Manga and European comics all at once.