Xbox360 core: not backwards compatible, can't patch (and in some cases "fix") games, can't download new content for cames, can't download movies or game demos, limited save game space, controller not wireless. so, I'm going to rule that out.
Let's compare the Xbox360 Premium versus the PS3 20GB:
More HD space on the PS3 since Microsoft takes up 6GB I believe, leaving you with 14GB. PS3 takes up 2GB I believe, leaving you with 18GB.
If you want the (superior) online service for online games for Xbox360 that's another $50 a year (PS3 free).
If you ever want to have wireless networking for the Xbox360 you'll need a $100 adapter. You can get the same for the PS3 by getting the Premium version (also $100) that includes the flash/sd/memory stick readers and of course a lot more space for all the content you can put on either consoles. PS3 can also install linux.
If you want to recharge a controller at any point while playing that's going to cost you another $15, although that is offset by the PS3 requiring you to buy a headset (unless you already have one for PS2).
So, $450 Xbox360 if you don't want to be able to charge your controller while playing, $465 if you do. For $45 more you get more space, free annual online games (which after 2 years would move Xbox360 to $500+ already), more space for games on blu-ray, knowledge that every PS3 user has a HD and therefore will have more of those related features included by developers, and of course the ability to play blu-ray movies. For HD-DVD, that would cost another $200 (which would move the total cost for Xbox360 to $650+ for the first year).
I don't think it comes down to cost. If you want the superior online capabilities (right now), then paying $50 a year might be for you. That's a matter of choice, not money. In the long run it would be more expensive to have the Xbox360, especially after 2 years.
The Xbox360 also includes a VGA port to connect to a monitor, while the PS3 comes with a HDMI port. I think the VGA is more useful because component cables can provide 1080p, but an adapter ($15-30) would be required to convert component to VGA for PS3 to be used on a monitor.
I think the PS3 Premium is the better deal. I enjoy the wireless capabilities because I can move my PS3 around more. I think the extra space is probably not worth the $100 alone because you can technically add any laptop-sized HD that is 200GB or less for more space, but I like the ability to use cards for saved games with the flash/SD/memory_stick readers, especially if you already have a digital camera or a PSP that uses one of those memory types (PSP uses memory sticks of course). Also, the Wii uses SD as well so you could get 1 stick for both system saves (if the formatting is compatible...someone else could confirm that as I don't have my Wii yet). anyway, that was the deciding factor for me. I hope to use my Wii and Digital Camera SD card also for PS3 saves, as well as my PSP memory stick. Either way, you still get the $100 worth of wireless adapter for the extra $100 and 40GB more space. Oh, and the chrome finish or whatever that really doesn't matter.
Anyway, I find the included wireless adapter (worth $100), 44GB more space (worth probably $20, but $30+ buying a 40GB HD), ability to use card readers, more space for games, more support for Hard Drives, and ability to play blu-ray movies to be worth the extra $150 to play online games the first year. If you don't want wireless, card readers, and 40GB more space (instead of 4GB more), then it's only $50 more for the 20gb PS3, the first year, and the same price after 2 years. 4 years of xbox live pays for the ps3 premium. If you don't play games online, I suppose the Xbox360 is the better deal.
Sorry for the long post.