i actually posted a threat with questions about these cars not that long ago, because i'm grinding in b-spec and wanted to get one of them to add to my rotation because they're so awesome looking.
i'll end up with both on the way to getting 1,000+ cars in my garage, but something to keep in mind that the concept (because it's electric) can't handle endurance races. because of this, i bought the road car first. i was able to do some tuning to it (so it has more hp than the race car, but it also weighs much more), and it'll compete with and beat lots of cars. when i eventually get the concept, it will just be for short races by my new b-spec bob when i finally retire one of my bobs that has maxed on experience...
(also, the race car is one of the prize cars - i think it's a b-spec race but i don't remember which one. i think it's better than the road car. just something to consider if you don't have any of them...)