Arm vibrato is more consistent in my opinion, although there are many great violinists out there that use a wrist vibrato. Let me use an analogy that might help regarding which one you might consider tackling. I find arm vibrato is like playing miniature golf and putting your hands on the appropriate spot on the grip of the putter to putt. Wrist vibrato, to me, is similar to taking that putter and putting your hands way on the bottom of the club by the blade (where you putt the ball). This doesn’t give you as much control as having your hands on the grip, otherwise you would see a lot more people playing miniature golf that way!
If you are interested in learning more about arm vs. forearm vibrato, check out this vibrato violin lesson. Any other thoughts or questions out there?