I find them both interesting, but Vollenweider gets a bit too new agey for me. Oppermann is very, very innovative and not afraid to experiment.
If you like Oppermann check out the album he did with Park Stickney. The two of them really sound good together. Their styles are not the same, but they blend incredibly well.
Vollenweider does what he does very good, but what he does generally puts me to sleep.