My DH and I have been to South Africa (x4), Namibia (x4), Botswana (x2), Zambia (x1), Kenya (x2) and Tanzania (x2) - the latter two most recently in August 2012. Kenya and Tanzania are our favourites - in my humble opinion, definitely a must-see, especially if you are able to travel in August/September when you are likely to see the wildebeest migration. The wildlife of Kenya and Tanzania perhaps may be matched in Southern Africa, but it is the people and the Masai culture in these two East African countries that we find so appealing and which drew us back to them for a second visit. Absolutely go to the Mara and Ngorongoro but, if it were me, I would include Tarangire National Park (great wildlife viewing and beautiful scenery, with many ancient baobabs) and the Serengeti. However, where you go will depend somewhat on when you travel and how long you have.

Below is the link to our photos and trip report from our first (2009) visit to Kenya and Tanzania - perhaps they will help you decide one way or the other. CR

http://bert-and-bin.smugmug.com/Travel/Kenya-Tanzania-2009/9504315_zcbkVx

Agree Kenya and Tanzania are great choices, but just visiting the Mara (in Kenya Visa USD$50) and Ngorongoro (in Tanzania Visa USD$100 for US passport holders; USD$50 most other passports), are quite a distance from one another.

Figure out how many days you wish to be 'in-country' where you'd like to visit and contact a tour operator or outfitter in one or both countries to create an itinerary for you.

If you haven't done so, pick-up a guidebook as Fodor's "Complete Safari Planning Guide" or one each specific to each country as Brandt's Tanzania or Rough Guide Kenya. Both countries have 'elephant' parks, 2 in Kenya, 1 in Tanzania; others where you'll find black/white Rhino; still other with species not found elsewhere.

Know that prices are seasonal, from 'low' the rainy months Apr/May, "mid" Nov to mid-Dec and June (in Kenya); all other months including June (in Tanzania) are 'peak'.

Maasai Mara - The wildebeest and zebra migration will likely be in the Mara mid-July through Oct, make it late July to early Oct for best odds. Over a million wildes and zebra are a different sight from Southern Africa. If you care to spend extended time at the Mara River or if you hit it lucky, you may also see a river crossing of the wildes and maybe zebra. That's different from Southern Africa. Finding a spot to view the crossing with few other vehicles is tough, though. Hoping Kenya Wildlife Service toughens up and makes vehicles in attendance behave better.

Eland are more likely in the Mara than Southern Africa. Should you include Lakipia or especially Samburu in your itinerary, there are unique species there not found in Southern Africa: Beisa Oryx (but it is very similar to the Gemsbok in Namibia), Grevy's Zebra, Reticulated Giraffe, Somali Ostrich, and probably most interesting, the Gerenuk which often browses on its hind legs.

The Maasai culture, present in both the Mara and crater, is different from the culture of Namibia or Botswana. You can choose how immersed you'd like to get into East African culture, from a brief visit to a boma-stay.

The Ngorongoro Crater is a World Heritage site. The crater itself is a unique landscape unlike Southern African landscapes. Viewing the animals in this setting offers a picturesque alternative. There may be flamingos on the lake, which I doubt you saw in Southern Africa. Sometimes blue monkeys are seen there, which are not so easily seen in Southern Africa.

An East Africa trip that includes only the crater and the Mara is not common. Usually at least one other location in TZ is included (see Canadian Robin's comments above) and perhaps another park or two in Kenya. Expanding beyond the crater and Mara could further diversify what you'd see and make for a very different trip from Southern Africa. Venturing further, to Rwanda or Uganda, and you could include gorillas, something not found in Southern Africa. You can also see chimps in East Africa in several locations if that's interesting to you.

Reading your comment: "but don't care to see the same sights as I have already seen in Southern Africa," makes me think you might enjoy an entirely different destination more than East Africa. If you still wanted Africa, maybe something like Morocco or even Egypt. Or choose South Africa and minimize the safari part but emphasize Cape Town for its culture/history in addition to its natural beauty, and do the Garden Route, Wine Country, Great White Sharks, penguins, Cape of Good Hope, etc. There is quite a bit on this forum of "what to do in Cape Town." I am assuming you have not been to South Africa based on your original post.

Finally, as a longshot suggestion, if you want something REALLY different from your other trip, with minimal duplication of previous sights, consider Zambia (kind of between East and Southern Africa, at least according to the bird books) in Nov or December for the largest mammal migration on earth--the fruit bats--and exploding carpet of color in the remote wilderness paradise of Liuwa Plains, which has its own wildebeest migration.

I just thought of some more unique things in Kenya. You'd likely be heading through Nairobi.

Copied from another post.

There are unique things in NBO if you wanted to devote a day to them.

-Karen Blixen Museum
-National Museum
-Giraffe Manor (hotel, expensive) or Giraffe Center (visit for an hour or less, not as expensive) where you feed giraffes by hand
-Sheldrick Ele Orphanage--group viewing of eles bathing midday or private viewing with all the other private "adoptive" parents in late afternoon.
-Mzuri bead factory, better than just the name suggests
-Nairobi National Park, the only national park in a city. Cool to see wild in the foreground animals (even rhino) with a backdrop of the city in the background

If you want to get straight to safari, then skip NBO.

Mzuri bead factory, better than just the name suggests

The name actually has a typo of M for K.

Kazrui bead factory.