I've used an Exos 46 and 58 briefly - the 46 on some 3 - 5 mile dayhikes, and the 58 on a one-night trip (about 10 miles, total.) I ended up passing them along to some folks who wanted to try them, and I settled on a Deuter ACT Lite 50+10.

The 46 and 58 vary only in size and a couple of minor design features (the 58 has a split pocket on the front of the pack, the 46 has a single pocket - that's the biggest difference.) The packbag itself is very nicely done, with 4 outside pockets plus a lid; the "inside-outside" compression strap allows you to run it inside the side pocket, or across the top of the side pocket as you prefer. I always found it very handy to use the long front pocket(s) for storing my tent and tent poles; it was perfect for a Big Agnes Fly Creek, Copper Spur, or Seedhouse solo tent.

So why didn't I like it? Because, as I eventually figured out, this isn't an internal frame pack. It's a high-tech makeover of an external frame pack, but it's still an external frame pack. As a result, you still get a slight bounce in the load, and it tends to move opposite the way you turn (unlike an internal frame, that moves with you.) The suspension didn't impress me. The shoulder straps were fine, and it does have load lifters. However, the hip belt does not wrap all the way around you; instead, there is a section of hipbelt sewn to each side of the frame. The mesh back panel (the "trampoline") is supposed to provide the rest of the wrap.

That doesn't work any better now than it did when I put webbing straps on each side of my old American Camper e-frame pack. Despite the load lifters, I could never get the Exos "hipbelt" to support the load (usually 20 - 25 pounds) adequately. No matter how tightly I pulled the belt, after a mile or two, the load settled onto my shoulders and stayed there; my shoulders always were sore after I carried the pack. I thought it might be that the hipbelt was just a little too lightly constructed. So, I tried an Atmos 50 on a 3 mile hike; it has the same general design, but stiffer shoulder straps and hipbelt material. It didn't make any difference, the load ended up on my shoulders. (Also consistent with my e-frame days: those "split" belts never carried a load as well as the full-wrap belts I eventually discovered on my Camp Trails packs; the full-wrap was completely capable of keeping the load off my shoulders.)

Let me be clear: I'm not trashing the Exos packs (or the Atmos, for that matter.) They are high-quality, well-made, creatively re-designed external frame packs. They have a lot going for them. However, you have to be clear that you're getting an external frame pack; you can't expect it to perform like an internal-frame pack. (Any more than you can expect an internal to perform like an external.) So, in comparing the Flash and Exos packs, just understand that it's not apples to apples.

For myself, I've found I far prefer a traditional internal frame pack, where the hipbelt and backpad are integrated into the structure of the pack. The Deuter pack produced an immediate "wow" when I put it on; it reminded me of the feel of my old Dana Terraplane pack (about 5 pounds lighter, though.) I've had it on 3 dayhikes so far, of 3 - 5 miles, and I hardly know I'm wearing it. My shoulders have yet to become sore, the hipbelt is supportive without being constrictive, and the weight stays where I put it (usually on my hips, with some occasional shifting to the shoulders, then back to the hips.)

I can't speak to the Flash pack series; I've never really looked at them. A quick look at REI's website indicates it's $10 cheaper than the ACT 50+10, and only a few ounces lighter (and it looks like they save it in the suspension, so beware - though it gets high marks from user reviews, so I could be wrong.) The 50+10 is only about 150 cubic inches smaller than the Flash 65. Deuter also makes an ACT 65+10, which is 600 cubic inches larger than the Flash 65; REI carries it, but not the 50+10. The user reviews of the Flash 65 seem pretty positive, overall.

You can make an apples to apples comparison of the Deuter and Flash packs; I can't since I've never used the Flash. If you haven't already ruled out Deuter, you could always order both it and the Flash, and send back the one you don't want.