Suggested caliber: 9mm (this gun seems like it's originally designed for 9mm). It has a noticeable nap for it due to high bore axis, but not too bad. The 40 versions are definitely extremely snappy.
Trigger mode: Personally, I like the AS version (DA/SA). However, if you want a consistent trigger pull throughout, get the QA version.
Size: I lean more toward the fullsize. Better sight radius, 3 backstrap options vs 2 for the compact and just as concealable (about the same size as a Glock 19).