The difference will be marginal by all accounts. They're both older generation processors and they're not gonna run NASA's Mars missions....
For use with Premiere Pro, both would be ill-advised. Systems with these processors are all in the range of the 10% worst performing systems, meaning that 90% of systems perform better because they have better CPU's. See http://ppbm5.com/DB-PPBM5-2.php
Since both CPU's are soo terribly slow, the best you can hope for is some light editing of SD DV material if time waiting is irrelevant to you. It is too underpowered to edit AVCHD or DSLR material. Investment wise, they are lousy choices, since you know you need to do serious upgrading to at least a top i5 or an i7 CPU before you start pulling out your wallet for one of your suggested ones. Then you may also need a different mobo and possibly different RAM.
Do you realize that the two CPUs you have listed require completely different motherboards? The i3 does not even fit the CPU socket of a motherboard that's designed for the Q9550. Nor does the Q9550 fit the CPU socket of anything designed for the i-series CPUs. The two CPUs use completely different and totally incompatible sockets - LGA 775 for the Q9550, LGA 1155 for the i3-2120.
With that said, AE relies heavily on the CPU. And you will need a much more powerful CPU than either of those two to run AE comfortably. This is because of the relatively poor performance of both of those CPUs in Premiere Pro (as Harm and I found out): At best, a system that's based on either CPU ranked above the bottom 10% (D1), but significantly below the bottom quartile (Q1), of the systems in the PPBM5 results list - and then, it really needed a GeForce GTX series GPU that cost substantially more money than the current going price of either CPU in order to even achieve such a result.
ofc i know this CPUs are not the best to choice from and know the CPU sockets they support. Im new to After effects nd Premiere and stuffs and done some Clips/Videos on a Core2Duo processor for school projects, and thinking to make a little upgrade. Got a very tight budget so thats why i was choosing between the 2 CPUs mentioned. Idk which to consider because, the fact that the Q9550 has 4 physical cores to use rather than i3's 2 core 2 thread specs..So this what i need to know is...
PS sorry for bad english, btw i use CS5.5 AE/Premiere
Here's the problem:
Depoending on where you get the CPU from, the Q9550 can cost more money than a quad-core i5-3470 plus a motherboard combined. This alone makes any upgrades to such an old PC a total waste of money. Plus, the Q9550 is based on an older, now-obsolete technology as compared to the i3-2120 (the older architecture that the Core 2 Quad is based on is significantly less efficient - significantly slower performance per clock - than the newer Sandy/Ivy Bridge architecture of current i3s).
Moreover, don't go with an i3 if you must get a new motherboard: The total cost of such a combo is too close to that of a combo with the exact same motherboard but with an i5-3470 for its own good. At least the i5-3470 would get you into the "budget PC" performance category (between Q1 and Med. in ranking) while either of the CPUs that you originally listed would still leave you in the "needs major upgrades" performance category.