I am with you on the X650... I have one I am going to throw away. It is the nicest breathing regulator when upright, but go inverted and be prepared to breath salt water... it leaks due to the design of the exhaust valve. The 250 was not quite as good, but is dry in any position.
Diving_is_a_passion:As far as cold water performance, all of the regs you have listed will prove reliable and will last years with a little bit of care.
Of the choices, I prefer the MK17/X650, although I would change the X650 into the G250V. The G250V has recently been released, replacing the G250HP. It has a metal air barrel, meaning that it will breath with moisture helping eliminate dry mouth. The G250V is identical to the old Graphite G250. IMO, For SP, this is a step in the right direction because some of their old designs are better than their current ones. It depends on your budget, but the MK17/G250V will work for anything and has decent routing.