I have had the iS144 for about 12-months. This replaced my Formula 159. I have found that XXL slalom/10m needs about 3 or knots more wind than formula/11m to get going.
In both of their wind minimums, the formula feels more powered up. How ever, once the wind fills in to 12/15k, XXL slalom starts to feel faster, my GPS showes this to be true for me.
In really light thermals, I missed formula, so I was going to change the iS144 for either an iS150 or get another Formula.
I recently borrowed an 08 iS150 from my LWS for 3-weeks. I tried the iS150 with 9m, 10m & 11m. Its good with all sizes. Fins from 51cm to 58cm tested. Smaller fins give much faster top end and 58cm fins gave it "mini formula" feel. In non planing mode, the iS150 feels much bigger than the iS150 and its easy slogging with an 11m. Once planing, it still feels much bigger than the iS144. One thing I did notice was that the iS150 feels smoother over chop C/W the iS144,this is due to it having thinner rails. The iS150 footstraps are slightly more in board C/W the iS144 which I did not like, but other may prefer this.
I personally really like my iS144 with a 9m and 51cm fin and hesitated to sell it. So I opted to get a good second hand Formula 161 for 10m/11m and keep the iS144 as well. I bit of a luxury having two light wind boards during credit crunch times, but the iS144 and F161 are two totally different boards, both are diamonds in different areas and its hard to choose which way to go.
I guess if you want "one board" to enter both area's, the iS150 would kinda give you the best of both. But not Diamond, more ruby
As you already have a very good light wind slalom (iS122), I think the iS131/iS144 will give you way to much cross over. So you should really consider the iS150 as the only sensible option. Has I said earlier, I was really impressed with the iS150 with a 9m/10m/11m. It only needs about 2 knots more wind than a formula, is faster in 12 knots and above, and eats chop (for such a big board) and its thin rails make it jibe very well (again for such a big board). If I would not loose a lot of money on selling my iS144, I would definetly have gone with the iS150 instead of an iS144 and Formula 161. In the future I may still do this???
I just wish that SB had made the iS150 95cm wide as it could have been classed as a "formula" in the GPSSS.com formula rankings.
Sorry for the extra long reply, but some times there's just no simple answer to these questions