Eric is right... it comes down to cost. All the new F17s delivered this year were with the SW Mast. Carbon mast is an option.
As for Performance, Bob Curry tested the new rig for six or seven months, with help from Bill Gillespie & Mark Batchelor

Looking at the NA Results for the top four boats. 1st. & 2nd. Placed F17s were with the aluminum mast... 3rd & 4th place boards had carbon rigs. all these boats had 1st., 2nd., & 3rd place finishes.
Note of interest, the 4th place F17 was a totally stock 2002 F17 with a four year old frt. end spinnaker.

Consensus in the Class, the SW Rig has the same performance as the expensive carbon rig

Regards