Well, as an owner of both, who bought his WebSurferPro to do just what you want, let me try to help out...
The CPU in the websurfer is slower. It has less ram by default, and it's a bit trickier to get both HD and networking on it. But, like you said, it has the audio out jacks already, hooks into the TV for video and has a nicer keyboard. Plus it looks more like a normal piece of stereo equipment.
The I-0pener on the other hand, has a faster processor, which can be more readily upgraded for faster speed. It comes with double the ram. It's a bit easier to get a distro of Linux onto the Sansdisk than it is on the WS PRo's DOC. The audio doesn't have jacks, but there are good (and rather simple) hack instructions available on how to add line outs for a stereo. The only downside is that networking is pretty much limited to USB (or parallel).
It all really depends on how you want to do it. I was planning to run the WS Pro with Linux booting off the DOC and having it mount my MP3 directory from my linux server via NFS. Then I can just play through a PCI NIC. The WS Pro's cyrix chip is much more flaky with the USB ethernet eating the resources than the I-Opener's Winchip is. I'm currently running into a heck of a time getting Linux to boot off the DOC though. So, YMMV.
Also, there have been numerous issues reported with IRQ problems on the WS Pro, along with heat buildup/ventilation. Be forwarned. There also is a much larger I-Opener community (just cause more people got their hands on them before Netpliance started cracking down) than there is for the WS Pro. This directly effects how much support you're able to get on the BBS.
Hope this helps a bit.