The screen may be giant but it won't take up the whole screen. And sound may be good but maybe an extra 5 feet top and bottom of the screen and side to side with a little more pristine sound isn't worth 6 bucks. Also, since it is an animated film, my guess it would look a lot better on a digital IMAX screen (LieMax). It's about the same price but if you get a student discount, at some places, it's just as much to see it on a normal screen so you might as well get that extra clear picture (which is the same size of a normal screen, just moved closer to the audience and bent on the sides to appear bigger) and good sound IMAX is known for.
Personally, I'm not even going to see the film in 3D because I don't see the need for it.
I saw the TS3 IMAX 3D trailer playing in front of Iron Man 2 in IMAX. The trailer wasn't in 3D (we had no glasses since IM2 wasn't a 3D movie), it was just among trailers for forthcoming IMAX features. Seeing it on the IMAX screen, I immediately thought, "This film doesn't need 3D if it's on the IMAX screen." Of course, this from a guy who's not particularly sold on 3D as a must-have movie feature. But I thought it look awesome in IMAX.
So given a choice, I'll always opt for IMAX rather than a standard theatre. Not only is the screen image larger but the sound system is amazing. And like dth316 said above, an animated feature looks crisp and sharp on a digitally projected IMAX screen. I honestly wished there was a non-3D IMAX screening, but I'm stuck with the 3D viewing. If you're iffy about paying the premium price, consider a matinee screening. That's what I plan to do, to see it Saturday or Sunday morning for a couple bucks cheaper.
Some of the AMC theaters here in Phoenix have digital 3D theaters, AND a converted IMAX 3d theater (it was an AMC stadium theater previously and was converted to show IMAX). Prices for evening shows are:
$13.75 for Imax 3D
$12.75 for 3D
$9.75 for no 3D
Really, I'm not sure it the IMAX 3D is any better than the regular 3D. Don't the IMAX screens have more of a square screeen? And really, $3 or $4 more to see a movie in 3D? Most 3D movies I've seen, just aren't worth the extra bucks for the limited scenes where 3D makes a difference.