There are no other wheel pilots currently available other than Raymarine. The S1 WP is a great Ap and it has beat the pants off every previous pilot I've owned including bellow deck units from Simrad & Raymarine. The only difference that I can see between the S1 and the X-5 is that the rate gyro is now built in but you can always add a rate gyro to the S1 making it not as outdated. The drive unit itself is unchanged, as far as I know, and it was unchanged from the ST 4000 Plus MKII when they went to the S1..
I run the S1 on a 36' 17,500 pound boat and it steers it very well with not even a sign of it being under powered.
Dave's Marine Electronics has the S1 for $1087.00
Defender has the S1 for $1059.00
Port Supply is wholesale only but they have it for $1089.00
The Defender price is very, very good!!