is there really that much of a difference between the 40 and the 45? i mean obviously the 45 is bigger and probably has more force but how big is that difference and is it really worth it? thanks

Yes and no. Its a double edged sword if you ask me.

The wise-arse answer is 5/100ths of an inch, but thats likely not what you are looking for. The real difference is in velocity. This is where you have to subscribe to a school of thought...do I want smaller/faster or larger/slower? Just like deciding on a rifle bullet caliber or arrow weight. The faster one will be flatter shooting, have more bark (and in this case more recoil too), or do I want something with a little heavier bullet, larger wound channel, but slower and not as flat shooting?

That depends entirely on which direction you are headed and what school of thought you are now a member of. I study at the school of 230. (ie: .45). I prefer a little bigger, slower bullet. Never had to try it out on two legged targets, and Lord save me I hope I never do either. I just like having a little extra diameter to make up for my personal inaccuracies with a handgun...as if 5/100" will do that anyway, but it helps me sleep at night.