You should probaby research the 4.0 firmware and it's added native features and see how important they are to YOU. Asking the rest of the world what's best for your needs doesn't make sense.
If youre perfectly content where you are, theres no reason to change it
If you want native mutitasking and wallpapers, camera zoom, and a couple of other little additions Apple included, go for it. Think about if those things interest you. If I didnt have a 3G, Id go up to iOS 4 cuz I enjoy it a lot on my iPod Touch 2G which doesnt lag and freeze on iOS 4 like my 3G does.
Its really up to you. Some stuff still doesnt work on iOS 4 in Cydia (a lot has been made to be compatible now tho) so check what Cydia items you use and see if they are compatible on iOS 4 too.
Personally, it's not worth it for me. I've been back and forth so many times between 3.1.2 and 4.0 I've lost count. Everytime I end up going back to 3.1.2 due to incompatibilty with jailbreak apps and Apples crap multitasking implementation. To each their own though, I just think 4.0 blows when compared to jailbroken 3.1.x.