Hi Alex,
Depending on your weight and if you are comfortable using a board that sinks when there is no wind, the SP44 would be the one to go for. I run it comfortably with 6.6m down to 4.4m. I've even used it with a 7.2m. It flies through lulls and is surprisingly easy to get going due to the relatively drawn-out rocker. In the past few months its been my most used board- we have been getting a lot of 18-25kt wind- perfect for 6.6m... Farrel O'shea set his Kirby PB on this board, and I've got most of my PBs on it (not much wind in the last year to really fire up the SP40 apart from one nice rain squall!). You would get a lot of use out of this board if you often sail powered up on 6.6m and 5.8m. It handles lake chop exceptionally well. It only starts to come unstuck in large swells (open ocean) with steep cross chop -ie very difficult conditions for any speed/slalom board. Gybing gets tricky in steep chop, but is easy peasy on flatter water and rolling swells.
The larger SP47 and SP50 are more suited to larger riders or as a board for really fast blasting in light-moderate or super gusty wind. They are a little bit harder to control on a proper speed course or really windy day (obviously). Having said that, Spotty (a bit of a mad gorilla

) nudged my SP50 over 46kts last year.
If you want a more relaxed overall ride in heavier chop (ie waist high+ and steep) then the slalom boards are excellent. I use these when sailing in the ocean or doing slalom- they are excellent boards through the gybes, and upwind, with solid top-end. I usually find myself a few knots quicker in peak speed on the speed boards though.
There are quite a few other guys around here who have the boards so they could probably give you an independent comment. My guess is you would be stoked with the SP44 to complement your other boards...