I'm waiting for the Browning Hi-Power in .45 GAP, but for right now, it's 9mm for me. Its the weapon I shoot best with. The M14 is a favorite, but for the kind of work that I'll ever see (urban warfare) it's not a great idea. My current longarm of choice is the Saiga 12 AK-based semi-auto mag fed shotgun. With lots of extra mags (but I still want more).
I shot one of those once and was very impressed. I figured it would pattern badly and have a lot of functional problems. They seem to be a great shotgun.
I would prefer the .45 ACP over the GAP. The GAP can't do anything better then the ACP, but the ACP can do more. Still, if you have smaller hands that is a good step up.
Me, I have fingers like sausages and huge hands. I like the old 1911 grips.