In my personal experience, iOS 4 was only marginally slower than iPhone OS 3, so it really comes down to features.
First, iOS 4 does not allow you to put home screen wallpapers on an iPhone 3G. I don't have experience with multitasking on a jailbroken device, but again iOS does not provide this functionality on the iPhone 3G. However, given the memory and processor of the iPhone 3G, you really wouldn't want to multitask on the device.
In terms of features that iOS 4 does offer the iPhone 3G, here are some top ones:
- Mail - unified inbox, multiple Exchange accounts, message threading
- Folders - a very useful way to store similar apps to save space
- iBooks - eBook and PDF reader (useful, though this app was somewhat slow on my 3G)
It's also worth noting that there are already apps that require iOS 4 and the number's only going to grow. Not having iOS 4 will reduce the number of apps that will run on your device.
Since I never really got a lot of utility from jailbreaking, it made the decision to upgrade pretty easy for me. However, if having a wallpaper or running apps from Cydia is important to you, you're not really missing out on a whole lot, at least for now.