The 6M is the same thing as the 6B and the Trane TWB. The top model is the 6A which has a slightly nicer cabinet, 10 year compressor & coil warranty and full compressor cover.
The Ameristar over the 6B which has the American Standard logo has some differences in perks such as co-op advertising, points towards trips for the dealer etc. That helps lower the cost to the dealer but means nothing to the homeowner.
There shouldn't be that much diff in price. Check out the bids carefully.
your not getting a sound blanket, a 5 year instead of a 10 year compressor/coil warranty & a different air handler
go with the american standard over the ameristar if possible you will be happier long term . it will cost you, you get what you pay for.