Tiger, I think, would be best optomized for the G5 line. Panther is great for G4, so, you may not want to upgrade to Tiger unless you have a G5 system. I don't think that there's going to be a larger difference other than the G5-64bit enabled Tiger OS, and a few other features, designed around the G5 platform.
Just as Panther took advantage of the G4, I think Tiger will take advantage of the G5. Simple as that.
I also figure that Tiger will take more advantage of cross-platforming. Microsoft is coming out with a virtual PC that takes advantage of the G5, and, I'm sure Tiger will be desigend to run smoothy with the New virtual PC. It's all gearred towards the G5.
Longhorn is designed to take advantage of 64bit technology -- the chips are out, but they are expensive, and nothing in windows takes advantage it, yet. It isn't coming out for a while; mostly because there isn't a high demand of users rushing out to get 64Bit xeon chips, or intels dual core hyperthreading thingamajjig.
Apple is far ahead in having G5's and now Tiger -- Microsoft may not launch longhorn until 2006. Mircorsoft they could launch longhorn this year, but again, there's the lack of 64bit PC machines...thus the lack of demand.
Apple has G5 sales, so there's a demand to sell a OS that'll really use the G5 core more completly than Panther. G4, however, is still popular, and it's selling more than the G5's, because of cost. So, I think Panther will still be around, and still be supported. Tiger will probably work on the G4, but you wont be using Tiger for all it's worth until you've got G5.
I say, G4 is good for normal stuff. G5 is good for high-end stuff; like, if your heavy into video/graphics encoding, use a G5. G5 has a big punch, but the average user hardly uses those punches, yet.
And windows, it may not have a lot of punches (not as much as apple), but is sure has a lot of blue screens.
I switched earlier this year, and I haven't gone back. I gave away my PC, actually. I have virtual PC, too, but I never use it.
My G4 powerbook is good for me, and I'm sure that I'm not using all of what G4 has to offer. But, it works and it runs, and I don't have to fiddle around forever trying to get the computer to work. I turn it on, and I do what I need to do.
Plus, I love to laugh when my friends (window users) have yet another crash, or have yet another virus (like the sasser). I like it when they come on over, and gawk at my screen -- my big wide screen on my Powerbook. The speed, and the smooth operation of the OS. And when they see what apple can really do, and what they can't do.
I think it's funny, everytime I hear someone say that macs suck -- I ask them why, and they think that a mac is like an atari. Then, I pull out my Powerbook and show them my fancy atari.
After that, they want an apple too.
--nate