interesting thread...cool that I have had both of these engines and MANY others..
really a toss up between the 2, the jx has more overall power but the v spec with a jp-3 has more bottom grunt, fuel mileage is similar unless you run a 2050 then the v spec trumps the jx. both engines are VERY dependable, I have a new version vspec blue head but updated carb and bearing I guess.
I put 8 gallons on a jx and currently have 4 1/2 on the vspec with no issues with either one. If I had to narrow it down to one then it would be the blue head updated v spec with a 2050 pipe. as far as price to rebuild I have no idea, the only motor I have recently rebuilt was an RB and thats just for practice.