I like O gauge for under the tree. I agree with C.W. Burfle, passenger trains look the most nostalgic in my opinion. Are you thinking of a newly manufactured tinplate set, or something vintage? If you go with something red and green you can't go wrong.
This was my under the tree layout from 2013. Its all American Flyer from the 20's-30's. These are the Ambassador cars, but there are less expensive green and red sets available.
Greg Northwoods Flyer