Shimano Ultegra 6700 10 Speed Cassette
The Shimano Ultegra 6700 cassettes feature a large selection of gears to optimize your everyday performance. The weight has dropped slightly by further drilling the rear cogs. In addition the individual teeth have been machined down to near perfection to improve shifting under load.
I replaced an Ultegra 11-25 with this. Seamless, easy swap--runs great (though I'm still adjusting the derailleur).
Replaced the factory Tiagra cassette with this unit and have benifitted with lighter weight, way better shifting. All in all very pleased and would buy again in the future.
For 'cross I won't go any higher than Ultegra (most often only 105...) but the Ultegra 10s cassette in a 12-25 is perfect. Inexpensive, considering I have a number of sets of wheels, and fairly durable.
You must get the Di2. I had Sram before...never going back.