The trispoke is the best choice in the front if you're facing variable wind conditions.
you need to understand Yaw and it's relationship to aero. Aero wheels don't give you the same watts savings at every different place on the course, which is dependent on wind direction and the speed you are travelling at. See the HED website for an explanation for yaw angle and their yaw angle calculator. Which is why Knowing the wind conditions at your start time is critical at the pointy end of things to do a really good TT.
Different companies also have different design philosophies with respect to yaw. baseed on this test: http://triathlon.competitor.com/files/2 ... Vision.pdf
Zipp has designed their wheels to dominate at low yaw, but stalls out earlier than the others- notice the graph when the drag increases sharply after 20 degrees. ENVE, HED and the bontrager aeolus aren't outrightly as aero but they will still mantain their performance after 20 degrees of yaw. whether this is a good thing for you will depend on your course conditions.
Back to the trispoke- according to HED ( and other) tests: (http://www.hedcycling.com/h3/default.as ... 3_Clincher
) it has the most 'consistent' performance across yaw angles, so if you have a technical TT with lots of direction changes, wind blowing all over the place.... the trispoke may be a better bet than a deep wheel like the 808. it is a versatile wheel, which is why it has been around for so long. It is certainly not the most aero, but if you're not sure, it is a good bet. I don't own a trispoke, so I can't comment on the handling benefits of putting one up front compared to a deep wheel. as mentioned, the shallower the wheel, the less affected it will be by crosswinds as well.
the disc is ALWAYS more aero on the rear than the trispoke, having a disc on the rear can add some stability in crosswinds as well. you will probably only ever see a trispoke rear selected over a disc in races where discs are banned ( e.g. Ironman world champs in Kona,) or on the track.
The teams are NOT moving away from the trispoke- look at this year's Tour TT pictures. Sky, Cannondale, Katusha, Astana, all rode trispokes in some form or other.