I don't have a lot of experience with the Dahon Matrix, but I have personally owned a Bike Friday Pocket Llama (same as Pocket Expedition, just the custom version), and we carry Montague at my shop, i've had a few good rides on those and some experience with them.

Firstly, from what I understand about the Matrix, it isn't a true off-road bicycle. I would rule it out right away for your needs, personally, if you're really looking for a true off-road machine.

Secondly, the Pocket Llama / Expedition rocks. Its tough as hell, has proper tire clearance, and can take true cross country level mountain bike beatings with ease. The only shortcoming to the Llama / Expedition is that when you start to get past intermediate difficulty terrain, particularly rocky and rooted areas, those little wheels have a tough time, no suspension doesn't help either. The bike is one that demands a lot of skill, more caution, and a good angle of attack to survive in some off-road conditions. It excels however, in riding conditions which demand agility and don't emphasize terrain obstacles (tight switchbacks, forest riding, etc, the Llama / Expedition will absolutely dominate)

This bike also has really great provisions for being a commuter bike. Rack and fender mounts, high quality, high payload racks are available from Bike Friday, the Llama can carry a boatload of weight and not break a sweat.

The Pocket Llama / Expedition has a few downsides:
1. Clumsier, large folding compared to most other folding bicycles. The Bike Friday pocket series bicycles are all very strong and well designed, but the folding system is not the most convenient out there. This isn't a big deal for someone who occasionally folds the bicycle, but I wouldn't buy this bike for an every day fold. You can get this bike into an airline legal suitcase if needed, it just needs some wrenching.
2. 20" Wheels and no suspension track through most off-road conditions well with a little finesse, but aren't optimal for larger ground obstacles

The Montague bikes are an improvement over the Matrix, IMO. The frame design allows for more real world off-road abuse, and they're pretty tough. I have had some fun on the Paratrooper and X70 models off-road once or twice each, neither one has been a poor experience. The bigger wheels and front suspension do indeed help with terrain obstacles. They're pretty reasonably equipped, although i'd avoid the cheapest model (X50), as it has some very low quality components on it.

There are 2 downsides to the Montague bikes that I consider important, personally:
1. Clumsy, larger folding. The Montague doesn't pack down small, and the front wheel is a separate item from the rest of the package when folded up. Again, I wouldn't get a Montague for every day folding, it isn't convenient. This one is really big when folded, one of the least compact out there.
2. Inferior provisions for carrying cargo. The Montague bikes don't play well with a lot of styles of racks if you want to fold the bike, you're stuck with things like seatpost racks and they're not much good for really carrying anything serious. I consider this a pretty big downer, personally.

Another option would be to call Bike Friday up and ask about their Pocket Gnu. They were still making it last time I asked, although it isn't listed on their website. The Gnu is an Expedition / Llama with a front suspension fork, improving its off-road abilities somewhat over the standard mountain models.