This is interesting. A few years ago I think it would have been Llanerch, no question. But the Tavistock renovation was one of the best I have seen. Between the tree removal and the greens being reworked it went way, way up in my eyes.
Both courses have some neat holes. The 18th at Llanerch is a great short par 4 and the 3rd is a good long par 4. The 14th at Tavistock is a great long par 4 and the 1st is a neat short par 4. Both have variety on the par 3s, and neither course has a lot of killer long par 5s, but they do have ones that dogleg nicely and some that play straight. You need to come out ready on the first four holes at Llanerch while Tavistock gives you a little breathing room early for the first two holes, and even four of the first five holes. I think Llanerch can play just a little bit harder so I give it an edge, but not by as much as it had been. Tavistock really rose in my personal ratings with their renovation and I would play either in a heartbeat, but I think Llanerch has always been underrated and is difficult while being subtle, and I like that.
The courses are similar in distance, and in the variety of holes, with short 4s and long 4s (at least long for someone 61 years old), and the par 5s play somewhat alike. The old Tavistock greens used to have small rolls that were tough to read, very much like Llanerch. The new greens at Tavistock are much tougher than the originals.
Personally I put Llanerch in the very low teens on my list, which might be a little high compared to others. I have it behind courses like Rolling Green, HV, and Waynesborough but I have it above courses like Torresdale, Lehigh, Springhaven, Jericho and White Manor. I have Tavistock in the low 20s, but it passed courses like Riverton, Philmont, Kennett Square and Whitford with the redo. It moved up a lot of spots for me. Both are courses you want to play.