For the same price I'd definitely go for the option with two lights. The redundancy is a real plus and you can set one up as your 'be seen' light aimed for the distance with the other light aimed more for seeing road obstacles - or have one head/helmet mounted with the other on the bike.

And in the event of an accident the usual excuse of 'I didn't see him' would be less believable if you have two lights on at the time - even if a single light would put out just as bright a beam.