I've been in leatherette loaners before, and I gotta say I didn't mind it.
I wouldn't buy a car with it... but didn't feel 'cheap or plasticky' to me... i just know that if i was gonna spend all that money on a BMW I'd feel alike wierd not ponying up for a basic luxury like leather.
All comes down to preference. I'd say go into a dealer and check out both and decide for yourself.
But I dont PERSONALLY believe its THAT bad/plasticky