I carry 10mm: a Glock 29 and a spare 10-round mag in a Safepacker, and a Glock 20 in my center console (with 3 spare 15-round mags that fit either gun).
I wanted to get a 10mm handgun years ago, and at the time the Glock 20 seemed to be the best choice. It has worked out well.
My son carried, and used (several times), a 9mm handgun while he was driving a M1 Abrams tank to Baghdad in 2003. He found it "marginally effective at best." He also carries a Glock 29 now.