I have skied both Mammoth and several sites around Tahoe (Heavenly, Squaw, Alpine). There is not a significant difference if there is a good base of snow at both places. Mammoth tends to get more snow. Heavenly touts the largest ski-able area in the US.
It did always seem colder in Tahoe though.
Are you going on a weekend? Everyone from LA goes to Mammoth on the weekends. It can be packed. During the week used to be empty there, not sure I have not been in a few years.

Have fun. You will love whatever you choose.