How much of a difference is there in the recoil of a .40 and .45 caliber? How much depends upon the load?
This is going to depend largely on who is answering, but personally, yes I think there is a significant difference.
Typically, the 45ACP recoil will move the handgun more than a 40S&W, but the velocity at which the recoil distance is traveled is slower than the 40S&W. This is probably due to the much lower PSI rating of the 45ACP, even in +p loads. So while followup shots on the 45ACP take longer, repeated recoil is, in my opinion, much more forgiving to the hands.
I've got no problem firing 40S&W in small doses, but if I exceed 200 rounds a session, I do begin to notice fatigue and sometimes physical wear (blisters). I've never gotten tired of shooting 45ACP, or 9mm for that matter
Think of the 40S&W as a midrange musical instrument (trumpet) and the 45ACP as a lower band, bass musical instrument (saxophone or maybe even tuba).
[Edit]: I've seen plenty of people who claim they notice little-to-no difference between 40S&W and 45ACP recoils. To be honest, I envy these guys.
A .40's recoil is snappier than a 45. Shooting a 45 the recoil feels more like a push where the 40 moving much faster from the dead stop will will make it feel much the same as to how the weapon rises but it does it faster.
All that will depend on the bullet weight and what weapon is firing it.
I'm one of those people who haven't noticed very much difference, with a catch. With my Beretta, the difference isn't that bad. Then again, it is a bigger, heavier, gun. On my Taurus Millenium Pro PT140, the difference is really noticeable. To be honest, though, the difference hasn't been enough to make me want to switch away from the .40.
Most the people that don't like the .40 are people used to a 9mm or new shooters that don't know what to expect. The recoil is snappier than a 9mm but it's not like it's so much a difference that it's going to hurt you...