Originally posted by twitch Equivalence is absolutely KEY when discussing FF vs APS-C, I don't think it's possible to worry about it too much. What ClassA laid out a few posts above is the #1 reason why we should care about sensor size, and why there are so many who want Pentax to come out with a FF option.
The reasons to want full frame are: (1) better specs (2) better viewfinder (3) better high iso and dynamic range (4) a desire for easier ability to get more narrow depth of field. Now, full frame cameras tend to have better specs, but this is not inherent to full frame, it is just as easy to spec an APS-C camera at a high level. In fact, by all accounts, the Canon 7D focuses faster than the 5D MkII.
APS-C cameras have currently passed the current crop of full frame cameras when it comes to dynamic range and have improved considerably when it comes to high iso. Sure, the next generation of full frame sensors will be better, but the question arises how good is good enough? Iso 6400 on the K5 is really decent.
So, we are left with viewfinder and ability to shoot with more narrow depth of field. Oddly, that last item seems to be the thing everyone focuses on. I understand that there are certain situations where it is crucial to have as narrow depth of field as possible. I just don't think with most every day shots a stop of depth of field makes that much difference.