Underground have gone with the tri-fin and seem to be marketing an out and out race board, with long tuttle fins. Apparently they have also incorporated the angled deck pad (which has been seen on JP windsurf race boards) to make it more comfortable to hold the board flat.
this was a hot topic following worlds last year...here is my point of view
off the bat you need to think about the physics of the board. Forgetting about the kite, picture the rider on one side of the board going upwind in the straps on starboard tack (right foot forward). Now imagine looking from behind (rider is on the right side of the board moving away from you)
the high aspect ratio fins that are dominating right now create lift that is approximately perpendicular to the direction you are moving. This creates a force on each fin that goes from left to right when looking at the rider from behind...thus why you are able to go upwind. One of the main differences between tri's and quads (in my opinion) is how the fins affect the overall balance of the board.
Keeping the perspective (following the rider upwind...looking from behind) imagine the board is a seesaw with the pivot point at the centerline of the board. The fins are creating a moment that makes the board want to rotate counter-clockwise (creating leeward heel for the sailors out there). This is like putting weight on the left side of the seesaw and lifting the right side of the seesaw. The rider is standing on the right side of the board. Their weight is pushing the right side of the board down and causing the board to rotate clockwise. This is like standing on the right side of the seesaw.
so the whole goal of a good raceboard setup is to have these forces balanced properly. The goal is to have your board "lock in" with slight leeward heel upwind. This means that the lift force from the fins has to create a moment that fights your body weight. If the fins are too small the board won't lock in and if they are really small the board might start edging (like a surfboard). If the fins are too big then the board will heel over too much away from you and you'll eat shit every time.
ok...why did i go into all of that
consider a quad...you have 4 fins that are a distance off the centerline. The moment that they create is a function of the size of the fin (the force they create) and the distance from the axis of rotation. If you look at a tri...you've taken the rear two fins of a quad and moved them to the centerline of the board. This reduces the overall moment that the fin can create, since the distance to the axis of rotation is reduced. SO what that means is that it is HARDER to get a tri to ride with the proper heel angle upwind.
For the exact same reason (and for other reasons)...tri's are easier to ride downwind because when running downwind you don't want the fins creating a moment at all. This is the idea of the board feeling "loose and fast" downwind.
Also we have to remember that a tri will be more efficient because 3 fins will create less induced drag (fin tip vortex) than 4. BUT when trying to create "equivalent boards" between a tri and quad...the tri might require more total fin surface area than the quad because the rear center fin is less efficient at creating the required moment upwind. So you reduce induced drag from only having 3 fins, but you actually increase skin friction by having more surface area...I don't know whether they offset each other perfectly...but overall the 'gains'...if there are any, aren't signficant.
so my opinion
overall quads are better but harder to ride
tri's are easier to ride, better downwind, but worse upwind.
and since upwind is the name of the game....and anybody CAN learn to crush downwind on a quad with enough practice....i'd go with a quad.
if you want to reach super fast for distance races....go with a tri
sorry that was uber long-winded...but i hope it all makes sense for ya'll