If your current phone does what you need it to do and you have the willpower to resist any urges to upgrade, then save the $$ and contract renewal for the next generation device.

Otherwise go for the upgrade.
I have a 3G and my other half has a 3G S. Every time I use her phone for anything whatsoever I'm impressed w/ the overall performance of even the simplest tasks. (Example: last Friday I updated the carrier file on my 3G then rebooted it. While it was rebooting I updated her 3G S' carrier file and rebooted it. The 3G S came back to the lock screen before the 3G did.) I'm not eligible for upgrade until January 2010. Despite how impressed I am w/ the 3G S, the 3G does what I need it to do. That said, the five months from January till June will be a test of willpower... a test that very well may be flunked.