I own a few pistol caliber carbines;
Bushmaster Carbon-15 dedicated 9MM AR - it has a molded lower with mag well, so can't use it for anything else. Uses Colt pattern mags, AR ergonomics, accurate and lots of fun.
Beretta CX4 Storm in 9, very handy, fits great, uses Ber92 mags. Accurate, lots of fun, but trigger could be improved. Needs a threaded barrel too.
Beretta CX4 Storm in 40, very handy, fits great, uses Ber96 mags. Accurate, lots of fun, but trigger could be improved.
Just Right Carbine, originally in 40, uses Glock mags, sort of ARish, but not entirely.
Picked up the conversion kit for 9MM, but haven't installed it yet. Shoots well, fits well and easy to accessorize.
Special Weapons MP5/HK94 Clone - MP5 type platform with all the pros and cons. It is lots of fun as well.
Ruger PC9 carbine, uses Ruger P series mags. Accurate, light, handy, lots of fun as well. Sadly, out of production, but you can find them around for not too much $.
Uzi carbine, it's a semi auto Uzi -- not nearly as much fun as the FA versions.
Another thing I have is the 9MM Mech Tech kit for using the Glock lower, it's OK, but I don't like "giving up" my Glock lower.
So, to best answer to your question -- strictly MY OPINION -- They can all be fun, but the order of what makes it to the range the most often in descending order -- Beretta CX4, Bushmaster, MP5, Just Right.