An Ultegra 12-30 cassette to go on the Shimano RS21 wheel set I ordered on Friday. I had a new 105 12-27 cassette I was going to use, but I'm afraid of some of the hills I'm heading out on. The wheels are an upgrade on the original set - a Weinmann rim around a Shimano Tiagra hub. It's nice to have another set of wheels and tires to use on the mag trainer. Where I live there are 6 months of winter. So, it's probably a minor upgrade - more for convenience than better performance.
If the hills are manageable, I might put the 12-27 back on. I love close gear ratios for shifting and keeping a nice cadence in all situations. When I raced criteriums in the early 1970's, I had a 49-52 chainring and what in those days we called a "straight block:" 11-15 (5 speed.) The races were flat and FAST!