Big advtange would be the 12mb (9550) cache over the 6mb (9400). They are pretty much the same in most other respects (yorkfield).
Remember that these have low multipliers, so you'll need a mobo that can get pretty good fsb. My personal fav right now is the Biostar TPOWER I45 - for reference, I have my Q9300 OC'd to 3.7Ghz with a 494fsb (roughly 2000Mhz Fsb) with a vcore of 1.28 at load (load temps are a little high at 66C, but that is about to change when I go to liquid) - high fsb potential (known to reach 600), full atx crossfire, and less than $140.
The board could go higher and I might try for 4.0 when I get my liquid cooling setup - but I'm thinking I'm pretty darn near the ceiling for these chips.
Forgot to answer your question: IMHO - It's only worth the +$50-$75 more if you do a lot of video editing/coding stuff. For today's games, the most important factor is the GPU. You'll see most noticable performance gain around 3-4 MB cache over 2 or less, but anything beyond 6-8MB cache and it's purely crunching numbers on a full load basis.