Best to get both. Early flamethrower pioneer is amazingly useful, especially against soviets where the cons need to get up close to beat any of your grens. It's also the only reliable building clear besides a mortar in the very early game when building rush is must important. Get a second pio for sweepers later, usually after you get a pak out and are holding. Ostheer is pretty defensive early game, so you should be holding and not be pushing deep enough to need the sweepers. Faster repairs from having 2 pios and especially getting vet on the flamethrower squad will be paying off in the long run. It's the gift that keeps on giving.
n.b. It's always map dependent to some extent. Flamethrowers aren't the best choice on crossing in the woods for example.
