I've experimented with a left hand mirror mount and on the MCL handlebar risers.
The big advantage of the left hand mirror mount was quick easy operation of the gps...my left hand barely needed to come off the left grip. But it was more exposed to rain & wind, and it vibrated some.
Center mount gps with MCL riser is not as convienient for at-speed-operation as left hand mirror mount, but it vibes less, less rain & wind on it, somewhat more shielded from sun so there is less glare. So my Gps stays in the center on MCL Ram ball. Also the center area gps mount seems to make for an easier eye scan of speedo, gas gauge, gps, and back to the road than the left hand mirror mount. It may just be me.
I considered a brake reservoir mount, but since most operations of a gps is with the left hand, it just wasn't going to work for me because of the need to reach all the way across. If you have cruise control the brake reservoir gps mount would allow your right hand to be free to operator the gps.
I know, shouldn't be playing with a gps while in motion..... usually just changing the zoom. Not like I'm planning a cross continental route on the fly.