Real:
I am sure there are others on this forum more knowledgeable than I. However, I offer my humble opinion for your use as you see fit.
I have the RCBS Precision Mics for .222 Rem., .243 WSSM, .308 Win., and .300 WSM. These are fine tools and will tell you a great deal about the chamber dimensions of your firearm. I do not have any of the Stony Point devices, so I cannot speak for their precision or usability, but the RCBS units do perform as advertized. Unless you have a large number of guns, the cost would appear to be nominal.
I doubt seriously the .243 and .308 are interchangeable eventhough they are, technically speaking, the same case. The headspace measurement is taken from the datum line which is near the mid point of the shoulder (between the body and the neck). Simple geometry indicates the shoulder for the .243 will be wider because of the smaller neck diameter and therefore the datum line will not occur in the same place. This would give an incorrect headspace dimension reading if you used the .308 in lieu of the .243 instrument.
Because firearms are prcision instruments, in my honest amateur opinion, it is rarely safe to substitute such things unless you are really sure of what you are doing. It is entirely possible you may get false readings that might disguise a potential problem or create a hazzard. I believe the $35 or so would be a wise investment. This really isn't the place to skimp on $ IMO.
I hope this is helpful.
BTW, I am trying to locate one in .260 Remington. Midway: nope. Cabella's: nope. Local shops: nope.
Any assistance would be greatly appreciated.
Poper