CSM (ret) Lamb has mentioned his preference for a sub compact double stack auto's in appendix carry in the past. The other options he mentions are good too. Pretty much a 4 minute knowledge bomb.
Incidentally Paul Howe carries a G26 appendix. John Mosby packs a Glock 9mm of some flavor appendix also. It is interesting to see commonalities in the equipment set up's of really experienced people.