I myself have the Sony DVP-S500D, and it's great. I've had no problems playing anything, and it's easy to use. The built-in Dolby Decoder matches up well with my previously purchased 5.1-ready receiver.
Others will have simular opinions on the Panasonic A310, but for my purchase the Sony had the deciding features:
* Plays CDR discs * Sounds better playing CD Audio discs (this is a fact due to it's dual laser design) * Nicer remote control
In therms of audio/video performance, you'll probably find both these models and their lower counterparts (without decoder, the S300D & A110) have very simular performance.
The Panasonic's one major feature is that it will pass a DTS signal on to a DTS decoder. Note the DTS decoder is not built in the player, so even if you get one with the Dolby Digital decoder built in you'd still have to purchase a seperate DTS decoder if you wanted to take advantage of that. The only problem is DTS discs are very scarce, and come at a premium price. Since I didn't want a seperate decoder anyways (no room) I went with the Sony.
