The S1 requires the rudder sensor, the X-5 does not. Apparently the new computer doesn't need it but it is available as an option. I don't quite understand that. Newer supposedly faster processing in the X-5 and it supposedly can "auto-learn" your boat during the initial sea trials. Of course sailnet has the S1 right now for $750. That's a sweet deal. There are a few places listing the X-5 for under $1100 but I don't know how trustworthy they are.
I purchased the S-1 this past fall and have yet to install it (I have not found the time and the biggest obstacle is installing the rudder sensor, not necessarily hard but time consuming just to empty the aft Lazarette to get access, LOL). Anyway, I called Raymarine and asked about an upgrade to the X-5 and they will upgrade an S-1 to the X-5 for $400 bucks (isn't that a coincidence). You have to send in the main computer unit only, I believe. I don't intend to do this, I got the S-1 to save some money in the first place and I think the rudder sensor is a reasonable approach to feedback to the unit rather than from the rate gyro.