See the big difference (as already mentioned here by some people) is that the PS2 was a pap dvd player - no decent remote control, poor responsiveness, terrible at reading dirty DVDs, poor UI etc etc. However the PS3 is one of the best blu-ray players on the market and due to its update able firmware, a CPU faster than your average blu-ray player and the a very good remote available it will be for some time to come.
The PS2 was my dvd player for a short while, basically i used it to confirm that DVD was infact that much better than VHS and then went out and bought a DVD player. The PS3 has actually replaced my current dvd player as i prefer the responsiveness and quick UI and i can see no reason why it won't remain my primary blu-ray player for years to come.