Pros and cons please or whats your thoughts and experience from going 3.1.3 to 4.1. Thank you!

iOS 4.1 is fine on an iPhone 3G so long as you do not enable multitasking -- the CPU really was not built for it and it will considerably slow the device as apps open. Aside from that, it's really no slower than 3.1.3. I prefer 4.2.1 as I feel I get better battery life on the 3G with 4.2.1 than I did with 4.1, and I also feel that 4.2.1 seems to run faster than 4.1. None of that is substantiated, it's just my experience, but the big one is just don't enable multitasking. It's not worth the performance hit you'll take.