IMHO the double-edge razor is the way to go for a lot of situations. A straight razor has advantages but they require some extra equipment, care in use and gentle handling. The old double edge is pretty much a tank. And it gives a nice, satisfying shave.

Probably the double edge will soon be gone. The blades are easily available at most any drugstore. The general quality of the blades is down. Used to like Wilkinson platinum because they were pretty much top shelf but they are no longer available. There are still good blades commonly available but to get the really fine ones you need to buy from one of the European manufacturers.

The double edged shavers themselves are available only in antique stores or you can get a German made unit online. Used to be any drugstore had a selection. The Vermont Country Store is one of the few places to get one. They specialize in carrying older devices and products.

If your into it it is possible to sharpen a double-edged razor blade. Back in the 20s and 30s there were quite a few proprietary devices that made it possible. Blades were expensive back then and people poor. I fiddled with a makeshift rig and, at best, I could extend the life of the blade for a couple more shaves. Only cost the tip of one of my fingers and a pint or so of blood. I figure I'm better off just stocking up on blades.

I tried the disposable razors but they seem like squirrelly little toys and I never like throwing out a plastic unit when it is just a tiny bit of steel that is dull. Of course the cheap disposable units are light. Something to think about if weight is a major concern. Disposables are handy to give to visitors or just to have a razor in a gas mask kit or glove compartment. Stuff a disposable, a sample-size shaving oil and an old CD for use as a mirror in a zip-lock and you have a cheap and handy shaving kit.

If you have plans to use a gas or filter mask you really need to be clean shaved. There are special hoods that allow our hirsute brothers, and Italian sisters, to stay safe but they are much less common and considerably more expensive. I understand that the Belgians had a gel that they kept in their gas mask bags that could be applied over facial hair to provide a seal in an emergency but I'm not sure how well it worked or where to get any.