Cyclocross - 105-10 vs. Ultegra 10 speed Cross

I am sure that this has been discussed but any info would be appreciated.
I am looking at 2 bikes that have essentially the same exact frame/rear triangle. One has an aluminum steerer tube, the other has a cro-mo steer tube, both have the same carbon fork, sans the steerer tube. I do not think that there will be much of a difference or that I will be able to tell the difference between the two framesets/

The bike that is cheapoer by a couple hundred dollars comes stocked with Shimano 10 Speed while the other comes with Ultegra 10-speed and dura-ace rear derail. They are both 2006 bikes. Is the Ultegra upgrade worth a few hundred dollars (300 to be exact).

I will be racing with this bike for a full season. If you have racing or hard riding experience with either I would appreciate the feed back.

Thank you.

Honestly, you likely won't know the difference once on the bike. The jump from 105 to Ultegra is minimal. If the Ultegra bike has better wheels or kit, then the $ may be worth it. Apples to apples, the 105 is the way to go.

I have Ultegra on my cross bike and it works perfectly. I'm also running a XT midcage r/d.

Also remember that cross is a very abusive sport. I had to replace a shifter last year due to a off camber 90 degree downhill turn in mud ( yes looking back I should have dismounted and run it). And I would much rather have to replace a 105 cost wise

Thanks!
I agree, replacing 105 will be much better on the wallet and I like to ride in the woods! Also, I have my own set of bars, stem post and a raceset of wheels, that are light and higher quality then the higher end bike. Deda stem and bars and either a Thomspon elite or carbon seat post, plus my SLR 135 saddle. So, the extra money on the Ultegra does not seem worth it, it would end up being over 300 more. The price is really good on the bike so I don't want to pass it up.

Thanks again.