What's your reason for upgrading? I think that would play a big part in your decision. If it's for better performance, I don't know how much of an advantage Tiger will have over Panther for non-G5 processors. I'm not saying it won't but I just don't know. I figured Tiger was concentrating more on G5 performance.
If it's for the features, look at what Panther has over Jaguar and what Tiger has over Panther. Personally, I think Tiger has better new features than what Panther gave to Jaguar (but Panther does have some cool stuff). If you couldn't care less about the new stuff coming in Tiger then you should get Panther now and start enjoying it. If you're going to get Tiger as soon as it comes out regardless, then you should hold off on Panther.