Shimano 105 or Ultegra?

I have the latest 105 on my commuter and test rode a giant advanced pro at the weekend with Ultrega. I couldn't detect any difference and I pretty much rode them back to back. If I could get the Defy with 105 and hydraulic discs I would choose that and invest in even better wheels.

The latest 2015 105 is fantastic, there are much better places to spend your money than going Ultegra- it depends on what else you are looking to buy and how much the weight difference makes to you

Just to add, I have the 105 and it is brilliant, snappy gear changes and brakes that actually stop you (unlike the no name one before )
I honestly can't see how much better 6800 could be.
Maybe spend the extra £200 on a basic training plan to get the most out of you.

I went with a mix. Ultegra chainset, fd, chain and cassette. 105 shifters and rd. Based on best value. Will only buy ultegra if it is less than 33% more than the 105 equivalent.
Yet to find brakes at a price I want to pay.
Frankly, don't think I would have been able to tell the difference whichever way I had gone.
Buy what you can afford and based on how important it is to you to have the name.
In the end it is down to what makes you feel best because it is you as the rider that has the biggest impact - by an order of several magnitudes.

Chorus is hands down superior. However, 11 speed 105 works pretty well!

However, for less money than Ultegra and more money than 105, you can get SRAM rival. Mechanically it's the same as top flight SRAM Red but without the carbon bling and lower weight. I run Force 22 on one of my bikes. It works very well indeed - the yaw FD is brilliant!

I hate the shifting mechanism on Shimano, but love the way it works on SRAM and campy.

If you are not using the brakes, go 105, from feedback from actual users, the calipers are the biggest difference.

Your Chorus is (was) a step up on both in performance and weight.