Personally, I like 2 x 12 cabs just because of the fuller sound. A 1 x 12 cab stacked under a combo will definitely give you a fuller sound, but not the same as a 2 x 12.
If you want portability, I would get the 1 x 12 cab or just keep my rig the way it is. If you want to add a bigger sound to your rig, I'd go for the 2 x 12. The only thought I have regarding either choice is how the speakers would match up with the AD50VT.
Guitars: 2003 and 2004 American series strats, Squier Classic Vibe 50's Strat, Squier Deluxe Strat.
Amps: Line 6 Spider IV 120, Vox AD50VT 212, and Peavey Transtube Bandit 112.
Pedals: Digitech Bad Monkey.