I'm a Pens fan, so there may be some bias...but regardless.
1. Who's better...
Now: Armstrong...by a slight bit. May be the fact he has amazing chemestry with Crosby, may be the fact he's playing with Crosby period, but he's playing pretty impressively all around right now. All facets of his game are already strong.
Future: Armstrong...by a slight bit. His game is more well-rounded...plus if he keeps developing a scoring touch he could legitimately find himself paired with Crosby for quite a while, he's already Crosby's preferred linemate. The left winger on Crosby's line seems to change every 5 games, but Colby has been there almost since the start of his stint in Pittsburgh.
2. Higher Ceiling
Umberger, definitely. Armstrong's ceiling was supposed to be third liner...Umberger's as a top 6 forward. Umberger has the potential to be a solid second line center who can play defense and score on regularity...Armstrong, without Crosby, was supposed to be a third line power forward with some offense...a poor man's Darcy Tucker.