Sorry to cut in on the SB team, but just read this and thought I'd chip in with my 2 cents worth
I have tested the iS144 and the iS150 back to back with 9m, 10, and 11m Warps. Here's what I learn't. Bear in mind I'm 105kg and 196cm.
*iS144 with an 11m will "not" go any earlier than a 10m. Too much sail weight on board and slogging is fairly hard going if the wind disappears. 9m and 10m are best for this board.
*In winds ranging from 10 to 20 knots (9m and 10m), the iS144 feels faster, is faster, gybes better and is generally more fun c/w iS150.
*In the winds that you state, 5k to 15k you will need the iS150 and at least a 10m and a grunt 64cm fin. In reality, 5 knots is not do-able, so 7 or 8 knots to 15 knots is more like it. In this lighter spectrum, the main difference between the iS144 & iS150 is that the iS150 will feel more "powered up" and better upwind/down wind angles c/w the iS144 which will be struggling in sub 10k winds. Yes, the iS144 will get going (with active slalom style ridding) in sub 10k, but its very physically hard work to do "all day long" and you will feel "under powered" with poor upwind/down wind angles.
*If your sailing in sub 10 knots most of the time, then your much better going with a Formula board and 10/11m. If your sailing mainly in 10 to 12 knots than iS150/10m is fine. If your sailing in 12 to 15k the iS144/9m is fine. Just depends on whether your in the up or lower end of the wind range you quoted.
As a guide for minimum winds to get planing, and ideal winds to feel fast and powered up, I'd go with this (but bear in mind there are lots of variables):
iS144: Min wind in 10m, 9/10 knots (feeling under powered and pretty slow)
Ideal winds in 9m, 12k to 20 knots, Feeling powered up, big smiles all
iS150: Min wind in 10m, 8/10 knots (feeling ok powered, much more powered than iS144)
Ideal wind in 10m, 10 to 15 knots, feeling powered up nicely.