If you are looking at mid level and above all of the groupos are good. The shifters are different and personal preference will determine which one you should get. There are some debates on the quality of shifting. I have heard from some that say that SRAM shifts quicker but Shimano shifts smoother. You can't reallty go wrong with Campagnolo, SRAM or Shimano.

Two things to consider in a cassette is the gear range and weight. The down side of a lighter cassette is that it probably will not be as durable. If you are racing, weight is more important that longevity but if you are a recreational cyclist, then you want your stuff to hold up as long as possible.

I am a Campy user and I like that it can be rebuilt. The down side to Campy is that it isn't available at every bike shop and you may or may not like the thumb shifter. Also, Campy will not come on a new bike. You will need to add it to a frame. Shimano and SRAM are available on new bikes.