I generally model in 1/48. I like to have all my subjects relative to one another, and the amount of detail in cockpits and such is usually good enough for me to build OOB. (I did get cockpit sets for my Blackburn Buccaneer and SAAB Viggen because I love painting cockpits and despise decal instrument panels in 1/48!) That being said, I do have a few airplanes that I really like, and they have earned a space in a 1/32 section. I have Hasegawa's F-104C in both scales in the same markings. I have built a few F-100s in 1/48 and got Trumpeter's 1/32 D just because I got a smoking deal on it. It and a 1/48 D will have the markings of "Triple Zilch" I Love the EE Lightning, so I have one in 1/32 along with an F-105. Other than that I'll finish William's Brother's series of racers in 1/32, because they have a nice set and at least they'll be relative to one another! I am however really tempted by the 1/32 B-17. It would be a fun build and the one thing it really has going for it storage wise is that it comes with a mount so you can hang it from your wall like a picture!
Now, I've also built a few 1/72 planes just because a 1/48 option wasn't offered...yet. The B-35, B-49 and B-36 are about it. Was going to do an XB-70, but Ken West came out with a really nice paper model in 1/48. (It's big, and includes a full cockpit!) I've just about got one done as practice, as I've never built a paper model, but it goes together fairly straightforward.
As far as why I model in 1/48, I don't know! I started young and that seemed to be the one that stuck! Now I'm just waiting on my 1/48 B-52 from Sanger!