The 360 and PS3 both offer good support for streaming multimedia from a PC.
However, you need to look at your own setup/needs to see which suits best
Codec wise, they are about equal, both now support most flavours of DivX/MP4/H264 etc, either streamed or from externally connected HDD's. Both certainly don't support all the exotic blends of subtitles and mutlichannel audio in a lot of video formats.
Both can use TVserity to stream various media, transcoding the ones they don't natively support. This sounds great, but for HD stuff, transcoding on the fly isn't a real option if you want decent HD quality, but SD stuff transcoded an a reasonable PC is pretty good.
The PS3 has a BR drive built in, which is a good source for media, and you can buy PLAY TV for TV on it as well!
The 360 connects natively to Windows Media Centre , and gives you the same look/feel of Windows media centre on the PC, and if your PC has a TV card, you get full control/streaming of that to the 360.
I think that's about it. when it comes to audio support, as long as you stick to MP3's, then both do it all..
That's all I can think of!