.40 is more readily available and cheaper. Personally I would skip both the 10mm and .40 and just get a .45acp. But if I had to choose, I would probably go with either a 10mm glock or a 10mm 1911... not a big fan of .40.
10mm availability has improved a lot recently. I've surprisingly found 10mm almost everywhere I've gone recently.
Local Rural King just got in something like 500 rounds of Armscor. I've also seen Federal, Am Eagle, and Hornady rather ubiquitously.
Bass Pro in north Cincy had a bad of Starline brass for 10mm that I almost bought even though I own not a single piece of reloading gear (yet). Starline's website has had 10mm sold out for several months.