Dude, it depends on what genre you play mostly. If you a variety of tones, go for the Valvetronic. If you're more into balls to the wall metal, the Cube is a better choice as it has a pretty sweet Rectifier modeler. You definitely can't get even a half decent tube amp with that budget bro, sorry. But dont worry, the Cube and Vox are both quite good enough improvements over the MG anyway. And they can get quite loud. But dont worry, you can always mic it up if you need to.

Once heard a joke that the MG stands for "Mahatma Ghandi" and the amp sounds like his stomach after going on anther one of those week long fasts of his. Haha, that was a pretty good way to sum up the amp. (Hope no one finds this offensive)

Yes, it is in your budget. And i'm not sure, but so far i've only seen it in CK Music. Although if you play metal and intend to get the Vox, you might want to get an OD pedal as a boost or something because the Vox doesn't really get very high-gain. But if you're not intending on using it for metal, its a good enough amp as it is.

NOOOOOO!!!!
He could do a LOT better than that with the budget he has.
As it's been said, it's his choice between the Roland Cube or a Vox Valvetronic.

All tube amps are teh sex! Hybrid amps are a marketing gimmick...