I've got the MX's and a few months ago moved up to the RX's, the SX's are mid range and for me personally, I've had no Issues with boths scales of the fox range, so yes, go ahead and get them Jon. Or if you're a tackle tart like me, get the RX's as theres not a better feeling than paying and exra couple of hundred quid for flashing lights!
I don't know if you're aware of this Jon but the SXV's have now been discontinued by FOX. Shop around/haggle a bit and you might be able to get some discount.
Again, don't know if you're aware but they work on a line vibration system rather than the more conventional roller as most alarms. If you're only fishing bolt rigs for carp (i.e. screaming runs only!) I'd go for one of the cheaper options but the SXV's are superb for very fine bite registration as the sensitivity can be turned up to unbelievable levels.
So I'd say if you fish for a variety of species, especially the shy biting sort or when you need immediate bite indication (eg pike) I'd buy the SXV (I've got 3) but for a 'standard' alarm save your pennies.
Have a look at the digital X3 and X5 range. Superb!