From my own experience I don't see a huge difference in real life. I was fortunate in that I found an TC-14 E III being sold locally dirt cheap, it was too good a deal to pass up. So I purchased it, honestly I can't say as I saw any difference at all using it in conjunction with my Nikkor 70-200mm 2.8 VRI.
Upside though I sold the EII for more than what I paid for the EIII - so it all worked out well enough for me. But unless you can get a really good deal like I did, I'd probably recommend you go with the EII