Great! What a wonderful choice you have to make!
Had you answered 'Hamburg' I would have recommended you to seek out the much rarer model C, but as you are in the market for an American Steinway, this is not an option!
One option you may wish to consider is a fully rebuilt model AIII, or 'stretch A' as they are sometimes called. No longer in production, the AIII was in between the length of the modern A and the modern B, at 6 ft, four-and-a-half. The modern A is 6'2.
The AIII had an even smoother bass/tenor transition than the modern A, and a depth of tone in the bass that was closer to the model B, and is considered by many people to be one of the finest instruments ever produced by Steinway.
However, if you are only considering new Steinways, then it is between the B and the modern A.
A very exceptional B can be almost perfect, but many B's seem to be just average.
It really comes down to the individual instruments in question... how many of each model does your local dealer stock? Perhaps your teacher can help you find the best one. Don't let the salesperson influence your decision!