While the 15GB is nice, I would recommend the iPod mini. First of all if you are expecting to jog with it, the mini is just that much smaller to make all the difference. Second, the mini might only hold 4GB, but it seems a lot easier to find songs and sift through a massive amount of songs. Lastly, the mini feels like it is built better than the 3G units.
I had the 15GB and sold it for the mini once I actually was able to use the mini an extended period of time (a friend of mine bought one). The mini is a far superior player than the 15GB. It would also help with the decision to wait or not, since they haven't even released the mini's in Europe yet, they shouldn't be updating them any time soon. The only problem would be finding one. I hope this helps.
Apmonia