I want to change from a 12-25 to a 12-27 9spd cassette. Currently I have 9 spd dura-ace cassette and chain. The chain/Cassette only has 900 miles on it, so I don't think I need a new one yet. Anyway, in looking at the prices for new cassettes, there is a good $50 difference between the ultegra 9-sp and the dura-ace 9-sp. What do you get for the $50, weight savings? If that's it, can I put the ultegra on and not have to re-adjust my der? or should I go with the DA.
[edited to add...SRAM is also cheaper than the DA, is that compatible also, or not?]

Also, I rate my mechanical skill at 2 on a scale from 1-5, with 5 being wiz, and 1 being totally inept. Is this a job I should leave to the LBS, or should I just get the tool and do it myself?