Both are awesome, though the Ps4 will be even better. I won't be getting it right away; not until a year after. I have a lot more to take out of the Ps3 yet. However, I will be very anxious to get a Ps4 in the meantime.
If a game comes out for both the PS3/PS4 then Id rather play it on the PS4. If its only a PS3 game I'll still have my PS3. I see no disadvantage to getting the PS4 from the start. Its not as if you buy a PS4 that you will be unable to use your PS3. Having two consoles is better then one, enjoy both.