I'm a NT of Scotland member, but the fun thing is, you get all the benefits of NT stuff for England/Wales as well.
Historic Scotland is great too. I was able to get my group of 4 into Edinburgh Castle all for free, which more than paid for itself. Next year, I will get all of Historic England places for free as well (which right now is a discount).
I think its nice to support these things and since I'm doing my fair whack of siteseeing, I think its worth it.