Deraillurs...is one really that much better than the other?

Depends on what "generation" the parts are.  Older Dura Ace will still be nice (assuming it was not abused and is in good shape) but if it is an 8 or 9 speed version the newer 10-speed Ultegra/105 stuff will work just as well and is more adaptable to new equipment.

If things were all new then you wouldn't probably notice that much of a difference in the performance of the derrrailleurs.  Where you'd notice it would be in the shifters (if it has the integrated shift/brake lever).  That and the brakes.

For me??  I always prefer newer stuff over used just because I am the one putting the abuse on the parts and I know what I've done on it.  I don't have to make assumptions of what the parts will have been through.  So I'd opt for the newer stuf 9 times out of 10.

Good article on the differences between Ultegra and Dura Ace (NOT DUAL ACE) derailleurs.

https://www.bikesportmichigan.com/features/dura-ace-ultegra.shtml

When you bring the "used" portion of the equation into play, all bets are off.

In the same model year, Dura-ace stuff (particularly the full kit) will perform noticably better than a 105/Ultegra combo.....all things being equal.

That said 2008 105 stuff will perform as well as, lets say, 2001 Dura ace stuff, and maybe better because it's new.

Hey again and thanks for the replies. I was going to give further info that the Aces components would be 2006's but then i read that very informatrive article on the differences between the two. (Thanks Bear for the link)

Since at my level and my goal of simply to finish the races in the top half of my age group, the weight saving i would get with the aces would not be a factor. I also liked the last sentence of the review,

"It is worth noting that few customers ever use a derailleur enough to actually wear it out. "

So durability isn't a issue either.

it dosent seem smart to buy the best used (with no warrenty) over very good new (with a 2 year warrenty). It looks like unless i can get a substancially better price on the used then getting the new one is a no brainer.