[quote name='Bencherstyle']is it worth it to play P3FES before P4? im planning to play one of them... don't know its worth it to play both.[/QUOTE]
Both games are worth your time. If you like one of them, you will most likely like the other one just as much. The games aren't for everyone though, so I would start off with one and if you like it, pick up the other. Whether that is 3 or 4, it is completely up to you.
If you are worried about story, the games take place in the same world, but do not follow the same characters or events, although there are some minor spoilers for P3 and lots of references to P3 (as well as other SMT games) in P4.
P4 made a lot of gameplay changes. Multi-mini-dungeons instead of one huge one, Player controlled party members instead of AI, and dungeon exploring during the same time as Social Links instead of separate, just to name a few. Whether they're improvements or unwanted changes is a preference thing.
I can't really recommend one over the other. I preferred P4, but it seems like more and more people prefer P3. I guess you'll just have to go with your gut.