Actually it's a difficult decision.. and it depends on it's usage with media..
Just plugging a HDD into or storing media on the device, then neither natively play a great deal.. Music formats are covered, video is lacking, DivX/XVid being the obvious.
However, you can install linux on the PS3, and run VLC/MPLayer or any other popular linux media player. The onlydown side is having to boot to linux, and use the utlilitarian VLC interface, it's easy enough if you use PC media players, but it's not family friendly.
It often works out more flexible to 'stream' media from your PC to your console..
The first level of this is to use a good uPnP server running on your PC, for example, TVersity
This allows you to play more media formats (XVid/DivX for example) through on the fly transcoding, and both console support these with little real issues.
The downsides are that the console uPNP interfaces are quite clunky and utilitarian, they will do the job, but not always the easiest to navigate, and some media needs fiddling with to get it to stream properly.
On this level, since both consoles will stream the same type of stuff, the BR player on the PS3 would make it slightly better.
Going one further, if you installed Win XP MCE or Vista Home Premium and used the 360 as a media extender (despite you not wanting to

, this will give you an uber slick media front end on your console, and with a modicum of setting up, you can stream pretty much anything you want to the 360, all indexed beautifully with nice menu's, even my 4 year old can use it!..
I'd not recommend either as a stand alone media 'centre'.. the PS3 with Linux would be the best solution, but it's no where near family friendly.. there are better standalone media streamers for £150 or less that would do a better job (other then BR/HD-DVD obviously)..
I personally still use my HTPC (£280 PC) sat under the TV, it's quieter then even the PS3, plays everything and using a free media front end (Media Portal), the missus can use it when I'm not there!... BR and HD-DVD drives are easily added, and it's upgradeable..
Some food for thought, if you give more details of what you would like it to do, we can probably offer more tailored advice..