RE: S-Type versus Carve
Unless you've got the killer boy racer DNA which will soon over rule the broader aspects of your windsurfing,(and require you to sacrifice all in favor of speed!) the Carve will likely be your best option as a fun all round choice for progressing intermediate use.
Carve is a more natual ride and jibe (ST good but more technical), and the generally easier going (although no snail..) nature of the Carve compliments progress, while not being too challenging. In lighter wind conditions, the C144 will also have a bit on an accel and early planing edge over ST137, which likes to be powered up that little bit extra.
On the other hand, if you see yourself smoking the locals on your new board this summer, maybe the ST137..
If you've any more questions, please let us know !
Cheers ~ Ian