I test drove a 2007 MCS about a month ago. When I got on a long entrance ramp to the freeway the back end seemed to twitch once to the left and once to the right. It didn't seem natural to me. The first generation Mini didn't do that. I told the dealer. I thought it might be low air pressure or bad tires. The dealer said they would check it; but they haven't got back to me. Any ideas? This has made me second guess the Mini as my new car.

 

Not enough info to tell. Could have been a lot of things.
1. Grooved road service (prolly not, as you say the r53 didn't do it).
2. Torque steer? Don't have a clue to your driving experience level... just sayin' is all.
3. Bad tire. The run flats have improved, but they do have a stiff sidewall.

I doubt the test drive was just the length of the long on ramp, but you isolate that it did it there. Did it stop after you cleared the ramp?

 

Could be lots of things. For example, you might have hit a paving groove in the asphalt. If the tire/wheel combo on the 1st and 2nd gen cars aren't the same, it will be a totally different experience across that groove. For example, a 15" or 16" standard tire might glide right over it where a 17" (45 profile) runflat could "catch and release" that groove and toss the car around.

 

doesn't seem to ever come up as a topic for discussion...I'd rest easy about moving to MINIs.