That's really the only way you could compare them. Although a more objective one would be a star test. And, that's the only worry....more mag' = more CA. Unless the optics were of very high quality the problem would get quite a bit worse at higher mag's. It'd be nice if he could get a 6" apo (a better comparison, really) but that'd mean mortgaging the house!!!
If he could get one, I'd be looking at an achro with a longer FL....that way the CA would be reduced.