I'm doing a Bspec endurance race at the moment so I can't check the figures for the cars, but from what I remember in this champiosnhip it's the AMuse GT1 Turbo that is the fastest AI car. The amuse has roughly 600bhp and weighs around 1100kgs., the Mne's R34 is roughly 600bhp but weighs over 1300kgs, I don't know about the other car you mention, but the Mine's does have a weight disadvantage.
Like I say, I can't check for exact figures, but the Amuse is the car you'll be battling against. There's also other factors to consider like tyres and driving styles / skills, these factors can have as much, if not more, affect on laps times as the natural difference in performance of these cars.
All I can suggest at the moment is buy the one you like, try the championship and if you loose, then slowly and surely tune your car until it becomes competitve and you can have a good race.
If I wasn't doing an endurance race I could check the figures and be more specific, but I can't, sorry. Maybe someone else can advise...
There is, however, another thread on what cars people have used for this race, if you can find that it may be of more help than me!