Durability, weight, shift quality, stopping power, component stiffness.

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Tom Demerly
The Tri Shop.com

Tom's right, but other side of the equation is that you're not going to hurt yourself by riding 105. I have spent nearly four years on Sora, Tiagra and 105, and my peformance hasn't suffered at all from riding 105. Most of us on this forum would do just fine on 105. But the other stuff is cooler to look at.

RP

As per Tom's comments. What I can add is I have a set of 6yr old 105 on which I raced 3IM (now on the winter bike) to a set of Dura Ace this year. 105 works just fine and can last a long time as per my experience, but the DA is pure luxury. In particular the shifting is so smooth compared to the 105 (even when the 105 were new)

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Simple Simon
Where's the Fried Chicken??

The components are slightly lighter which in the big picture of combined rider/bike weight is miniscule.

The only difference I've ever noticed in 105/Ultegra/DuraAce (owned all three) is the shorter shift in the DA STI which makes for crisper shifting. The DA rear der. is also said to be better. I wouldn't rush out and upgrade to DA. It won't make you any faster. If upgrading I'd recommend only bother with the shifters and the rear der.

I'd agree with most of the conclusions above.

However, I find the new DA cranks to be on another planet than anything else.

So, give me DA 7800 cranks and DA rear der (only b/c it sees the most use) and I'm fine.

I rode Ultegra for years and now use DA 10sp. It's nice. However, Ultegra is perfectly acceptable and performs brilliantly. I have no problem spending the extra money on quality, but will say that next round I will probably go Ultegra 10sp and save the money over Dura Ace. It is very good.