I had a FT138 and now have a FU133 (but not as a replacement for the FT138!)
I weigh ~ 85Kg
The FU 133 is a nice manouverable board but more like for 14 kts++, below that I prefer (unless close to 14 and very choppy) using my iS133 W85 with a 10.4 /9.5 or 8.7. The iS133 is my replacement for the FT138 and really does a good job.
if you want to go fast, plane early in 8-15 kts -> FT138
towards the lower range of 10-15 kts you will not really have that much fun an a FU133, sure its turny but if you aren't or just barely planing turny just isnt fun anymore...