Where should we visit. We have to choose between the two. So undecided.
It depends on what you like. Glendalough is a monastic settlement ruin with a round tower. There is hiking, if you like that, and a nice museum. Powerscourt (which is normally included on a half-day tour to the area, along with Glendalough) is a mansion with beautiful gardens. If your trip is in the summer, the gardens should be nice but even the "long" walking path only takes 30 minutes or so. If you are traveling during part of the year when the gardens would not be in bloom, there is little reason to go there, in my opinion.
Enjoyed Glendolough. Walked to the upper lake (about 2 Km) for additional ruins. Was a great view at what life was like 1000 years ago. Can't comment on Powerscourt.
Glendalough offers more hiking opportunities but if you love gardens Powers Court is lovely. A proper garden will have something blooming almost year round. We visited both places last October and everything that was blooming was spectacular. The old mansion there is mostly a burned out shell but has a nice cafe for lunch with nice views of the gardens.