Brother in law gave me these bikes, I was looking to clean one of them up to use for exercise/fun. Any advice on which one I should keep and can/should I sell the other?

assuming they both fit you, they’re very different bikes so it kind of depends what you want to do.

the langster is a single speed (or fixed gear, depending on the rear wheel setup). fast, fun, a very specific kind of riding, steeper learning curve especially if its fixed. i’m guessing this has an aluminum frame which will be lighter than the trek.

the trek multitrack is an old hardtail mountain bike. great for trails, gravel paths, has lots more tire clearance than the langster. you can run big tires, commute or bikepack on it. more relaxed kind of fit and more comfortable over longer distances.

I’d buy the Langster in a heartbeat.

The 720 has some retro value and is historically significant, but it’s a piece of shit that belongs in a museum. The road bike looks quite nice.

Keep both, transfer over the cockpit of the spesh to the trek, become famous on /r/xbiking for a day