the real life difference is that the E8400 and Q9550 are on a dead socket Intel will no longer support with future CPU. While the i3 is on a socket that will have future CPU released in the coming years. So the i3 would be your best bet for future proofing a new PC build while the Q9550 is fine for upgrading a current build.
The problem with the E8400 is if you want to upgrade in 18 months or so, prices for Q9550 will only be higher. Its a fine CPU though if you plan to make a new build and just want something to hold you over.