Patty

I haven't been to the Mara in May/Jun, only late Nov (also non-migration time) and thought that there was plenty of game to see, but maybe I'm just easy to please I went to Samburu in early Dec last year (during torrential rains) and also saw plenty of game there. It really depends on how important seeing the migration is to you. Have you looked through the East Africa trip report index compiled by Lynda http://www.fodors.com/forums/threads...&tid=34860283?
If not, look for reports/photos from that time frame and see if that gives you a better idea of the game viewing.

One benefit of doing an all Tanzania northern circuit safari is that you could do a driving (or one way drive) safari and this would allow you to have private game drives. The Kenya locations are further apart and on worse roads, so flying is a better option particularly with limited time, so you'd be going on shared game drives unless you choose to pay extra for a private vehicle (either through the lodge/camp or your tour operator).

ShayTay

Basically, the Mara is the northern end of the Serengeti ecosystem. The Serengeti will be your best bet at the time of year you're going. I would agree with staying in Tanzania for your entire trip. The problem is that there are fewer "beds" in the Serengeti, compared to the Mara, despite the fact that the Serengeti is almost eight times larger. Mobile tented camps in the Serengeti are an option; you might check on some that are set already set up there.

sandi

maasai - If you haven't noticed, these visitors plan to travel end- May/early-June. Their best destination is Tanzania and Zanzibar.

While Kenya is wonderful, the Masai Mara a favorite stop of mine, is not really a good option vs the Serengti at this time.