The diecast alone tends to make the BT's seem more worth the price, but as toys (elegance of transform and play) i think MP Lambor has it beat.
The Viper mould (fantastic as a car of course) is probably one of the least impressive of the BT robots - his beer gut is pretty ugly and proportionally looks all wrong cause of the tyres on his shoulders.
Surely the BT's feet and knees give you more trouble than MP Lambor's shoulders?! And are you seriously risking the beautiful diecast during play? Maybe the Alternator version.... :)