I've used dozens and dozens of friction control devices for descent. I would dissuade you from a fig 8. Yes, they work, but there are better, lighter, smaller and even cheaper alternatives.
Friction control with a device, IMO, should allow you more versatility, control and security than using a hitch, otherwise you should just use a friction hitch.
A device should allow the use of a single rope, or a doubled rope. In using a doubled rope it should allow you to either control both parallel strands identically (1:1), or control one of the two lines (2:1) with the other end anchored to your saddle. This latter method is simply subbing the piece in for a traditional hitch to handle your friction.
Regardless, being able to choose interchangably betwen 1:1 SRT, 1:1 DRT and 2:1 DRT is really nice, whether from climb to climb, or within the same climb. EZ on, EZ off is critical IMO because you often need to re-route your line around something to accomodate you, as opposed to re-routing yourself to accomodate the line.
So far, this takes the GriGri and the Stop out of the equation. Surprisingly, a figure 8 can still accomplish all this, but it twists your line; two-fold in a 2:1 system, but even the twisting in a 1:1 method, it's still a PITA. There is a way to use certain 8's non-traditionally that won't twist the line, but it really depends on the 8 of which one will allow you to do that.
Climbing on 13 mm traditional arborist line will limit you from using just about every device out there. The fig 8 will still accomodate this, esp the rescue 8 with ears. Even some of our under 13 lines, like poison ivy and Fly will either not fit in a lot of the devices, or you have to work to stuff them in, making the overall process of climbing more work and less swift than it could be.
Moving on to true 11 mm line for climbing (and using 13 mm ropes for lowering) would be advisable if you want to enjoy tree care off a device, not just A device, but any piece out there.
Last note. The word 'descender' gives the impression that it is for descending. Well, yes, but it certainly doesn't stop there. I like to refer to these pieces as friction control devices. Yes, you control friction while coming down, but you also control friction while moving around. In fact, you should best assume that while aloft, you need to be in control of friction 100% of the time, other than the times you are standing on a limb with the rope untensioned. ANY TIME you have tension on your climbing line, you must be in control of that tension, allowing yourself to hang stationary on the rope, or move down. This is true for hitch systems or mechanical systems. If you climb back up, you need to easily be able to pull rope back through the other direction (slack tend) to re-tension.
There are many devices that will allow you to do everything decribed above, all have certain nuances that are the performance differences between each other. Also, the ideal friction control device for tree climbing, since tree climbing on devices is still in it's infancy, has not yet been invented.
Does this help at all?