Borobudur
I can't help you decide on which but if you do choose Borobudur, keep in mind the following:
1. You can take an early AM flight from Jkt and be back by evening.
2. Get a tour guide with car for your visit, with airport pick up. It's not that expensive and will make a world of difference.
3. Djojakarta and Solo are the two cities/towns nearby and at least when I lived in Indonesia in early '90's, not much to see. You might want to wander up and down Jl. Marliboro, lots of hawkers and touristy things to buy!