^ i think the roland cube 30x has a lot more amp models than the 20 (if the 20 even has any). Considering the main boon of the cube is its amp models, if you ask me it doesn't make much sense to buy a cube without (or with very little) amp modelling. I tried the cube 15 (which has no amp modelling) a while back, and, to be frank, it was terrible.
http://www.rolandus.com/products/productdetails.aspx?ObjectId=821&ParentId=57# (cube 20) "TYPE Switch (ACOUSTIC, OVERDRIVE, DISTORTION, METAL, METAL STACK, TUBE DRIVE), " (those aren't really amp models, they're more like stompbox models, from what i remember)
http://www.rolandus.com/products/productdetails.aspx?ObjectId=822&ParentId=57 (cube 30) "TYPE Switch (ACOUSTIC, BLACK PANEL, BRIT COMBO, TWEED, CLASSIC STACK, METAL STACK, R-FIER STACK, DYNA AMP), " (those are amp models)
EDIT: i'd say the pros of the cube are the high gain tones, and the cons are the non-high gain tones (though i can't comment on build quality or reliability as i've never owned one). i know (laugh if you like) us regulars over at UG generally recommend the cube as a starter amp if the starter plays metal, and the vox if he/she, er, doesn't. :lol: It's more or less as simple as that (though obviously take personal preference into account).
The Super Champ XD is better than the Vox AD30VT and miles better than the Cube 30.
It doesn't have a headphone socket though.
ah, dang, i told you i hadn't checked about the headphone socket!
I still think i'd prefer the cube for out-and-out metal, but for everything rock and lighter, the 5 watt fender (still unsure which model!) is miles better. It's modelling, but it actually sounds like a tube amp too. :)