I voted combination.
For Trek books, I'm still 100% paper, except for eBook exclusives, obviously. (Well, OK, I bought the Destiny omnibus in eBook format, but that's in addition to the original paperback releases.)
For financial reasons, I've being trying to severely cut back my non-Trek book spending, and opting for the library instead. However, for the few non-Trek books I still do buy, they have recently started to skew about 80% eBook vs. about 20% paper.