In general, which performs better? How are they like when processing vector graphics?
Bubblemark is the premier benchmarking site for RIA.
Note at least one comparison image showing better drawing in Flash.
The GUIMark test is very interesting, the initial results showing SilverLight performance as poor. If you read down into the comments, the solution was identified as being partly a coding problem with timing and partly due to a major speed issue with SilverLight rendering text.
So, one key issue would be if text forms a major part of what you wish to render at speed.
Aside - I did a realtime graphing engine in Cocoa back in 2003 where I ended up using traditional Quickdraw rendering because the heavily anti-aliased and fancy text rendering of the Quartz graphics on Mac OS/X at the time was way too slow. Fast, good-looking text is not easy!