I am a big fan of the auto-block belay devices. The Petzl Reverso III and the Black Diamond ATC-Guide are the two I have used a lot.
I have bought my own Reverso IIIs and older Reversos before that. And bought as well as been given the newest Black Diamond ATC-Guide. It is not unusual for me to give skinny rope belay devices to my partners. Not everyone climbs on or likes 7.7 twin ropes. Since they are belaying me, I think giving away a high quality belay device it is a good investment. Generally I have given away ATC Guides because, I have been given several in turn, and the Petzl is more expensive and harder to find.
Hard climbs and skinny ropes in the future? I'll make a point to loan my partner a Petzl Reverso III ;-)
Rapping the Pencil or the Midi bridge makes you a believer.
I generally use either a a 9.1 Beal Joker as a single rope or a set of 7.7mm Beal Ice Twins. So a belay device that will work in auto-block mode, rapping and regular belay mode with thin ropes is important to me. For my use on the thinner ropes I think the Reverso works better than the ACT Guide when belaying off the anchor.
The direction of the clip in point is different. I find the Perzl much more user friendly. Bd's version clip in point is at right angles to the Petzl.
If you are using a rope fatter than a 10mm get the ATC guide. The Reverso gets sticky in auto mode as the ropes get bigger. 10mm seem to be the auto block limit on the Reverso III.
Neither company's belay device is very durable. If they actually anodize the aluminum it has to be the worse anodizing job in the world. As both companies belay devices wear quickly on the surface. So toss up there. Also don't buy the silver colored (clear anodizing) versions. The directions (if you ever need them) on the belay device disappear within days of first using the clear version.
Other belay device reviews that are worth a moment to read and a lot more comparisons on Supertopo gear reviews. More rock gear reviewed there than I will ever see, let alone use. FWIW they are reviews I trust to be accurate.