I've used both Pionner 16X DVD-ROM slot load and tray load (using the slot load right now).
Every single CD and DVD worked perfectly on both of them, but I have to say that the slot load is a bit faster (in access time) and makes less noise (the stabilization table in it seems to be higher quality).
Last, you have to know that there is a digital sound outpout behind the slot load DVD that can be connected directly to the sound card, but there isn't with the tray load. Overall, the slot load seems to be higher quality, but in the long run, I don't know if it will break (had it for 6 months).