Do you think it would be a bad idea to trade a S&W M&P .40 cal to get a "cheap" .45 cal?
David: It's a very personal choice. Carry what you feel confident and comfortable with. There isn't a huge difference in ballistics between .40 and .45.
Either of them are viable choices but the M&P brings roughly twice as much total firepower.
The way I see it, the ODDS that I (living in an ordinary neighborhood in an ordinary city, doing ordinary things) will ever need more than 8 shots fired in an SD scenario ARE TINY.
Statistically, most gun exchanges consist of 3 or less total shots fired. It's highly unlikely that a non-Leo ordinary citizen will ever be involved in a high-round-count gun "battle".
The (all-steel) 1911 will be heavier than the (some-polymer) M&P... but thinner.
1911's have so much going for them generally but I'm personally not a big fan of carrying cocked-and-locked (1911 style) so that's a factor for me.
An M&P .40 Compact (10+1) would work better for me as a carry but it's your choice...