Both can be flashed to an XT card with the correct 9800 XT BIOS, and either can typically be overclocked a good deal past 9800 XT speeds, with good cooling, and a bit of luck.
However, if you flash an R350 cored card with an XT BIOS, you will see some artifacting in a very few programs due to the shader that the R350 lacks, and the R360 has. The most common of these programs is the troll jump test in 3DMark 2003.
All-in-all, you can't really loose with either core so long as you don't get a bunk card. R360s with an XT PCB are generally the most desireable card, if only for the built-in temperature probe.
I've seen just as many heavily overclocked R360s as I have seen heavily overclocked R350s; although R350s typically clock up a little bit higher than R360s tend to with similar overvolts and cooling, the difference isn't signifigant, or any reason to change your purchase plans.