I have both a BGE and a Bubba Keg, they are both excellent products, though the Big steel keg is a better value
The keg is significantly more durable than the egg, its made of thick double walled steel with oven insulation in between, there have been no reports of the side tables melting. It comes with a 5 year warranty and has no parts designed to wear out like the felt gasket (it uses a soft metal mesh sealer like your oven), The BGE is fragile and is prone to cracking.
Control wise both grills are near identical, its just as easy to dial in a temperature on both grills using near identical controls.
The point about upgrading is moot, the Bubba Keg comes in only one size, which is the same size as the large Big Green Egg, incidentally all BGE accessories such as pizza stones or heat shields also fit in the Bubba Keg
Lastly the Bubba keg comes with everything you need, stand, cleaning tools, accessories in one package for $599, the same size grill from Big Green Egg costs $749 and doesnt include the stand ($125) or side tables ($60)