I think you first have to consider if the number of skin types in a skin suite mean anything to you. For instance, the Carbonix skin pack incudes only windowblind skins and their corresponding wallpapers.
The other premium suites contain several differnet application skins. For instnace, Solara and Abracadbra have Winamp, rainlendar and sysmetrix skins, (though the abradabra sysmetrix skin is free for download). PixOS includes a trillian skin and sysmetrix skin. There's a free wmp skin for it as well.
Check out each premium suite to see what it has and doesn't have
If you disregard that and look at them soley on the basis of style, they are all cool.
Abracadabra is pretty high tech looking. PixOS is really sleek and clean. Solara is nice and funky with its blue and orange colors.
Carbonix has 5 skins ranging from the really sleek and professional (Apogee) to over the top Cardz. Cardzz is the only skin in the Carbonix suite that you probably wouldn't use on a regular basis. Its more a show and tell skin. The advantage of this suite is you get multiple WindowBlind skins.
That said, if I had to pick one and only one, it would be PixOS. I end up using that suite quite alot. But it's easy for me to say that since I bought all the above suites
