I take Armstrong via clear decison over 15, closer over 12. At any weight. It'd be one helluva war, though.
I'd favor Armstrong as well - I think he's too strong for Chavez on the inside (given that he could outmuscle fighters that weighed as much as 15o-plus lbs.) and Chavez likely wouldn't be able to fight off the back foot well enough to win.
I would pick Armstrong at every weight (130, LW, 140, and WW). He may even stop Chavez at WW (any time after the long in the tooth version against Whitaker) He would also out-box Chavez Jr., regardless of the 40 lb. weight advantage