Nothing wrong with a 40 cal, except they're slightly more expensive to fire per round than a 9mm. I like the 40 myself and often carry a Kahr MK40 with the extended mag for additional capacity. Not real fond of the M&P Shield 40, I'd rather carry the M&P 40 Compact; I like it even better than the M&P 9 Compact. Haven't shot a polymer Kahr in any caliber, but if they're like their steel guns K & MK series, they're probably very reliable and accurate; though they are a bit expensive compared to some other guns out there. Another nice 40 though on the expensive side, is the EMP40. Bottom line, try out everything you might be interested in. See how they feel, is the weight an issue, how well do they holster and where will you carry it? I like something I can comfortably carry in multiple methods: wasteband, hip, ankle if need be. Most of the time it's the Kahr MK40, Sig P938 or LCP. I can carry any of these in all 3 methods comfortably all day. The M&P 40 Compact or the M&P Shield 40, only works hip carry or shoulder holster. I don't do shoulder except for during outdoor hiking with a more formidable weapon than those mentioned here. Good luck in your search, may the choice you make, be one you are happy with for many years and safe journeys.