Ruaha is superb. It has cheetah, which Selous doesn't, and, being at the crossroads of East and Southern African habitats has a huge mix of bird and wildlife.
It's about half the size of Selous, but that is misleading as there is no hunting - it's a park, so it probably has about 10 times the amount of area for tourists as Selous.
It's about 3 times the size of the Serengeti NP and gets a fraction of the visitors there. I once saw 7 vehicles around a lion and our driver apologised for teh crowds, and promptly drove off to find a pride with no-one else around. I'm not sure exclusive sightings are as common in the north.
However, I'd be cautious of combining the north (Crater, Serengeti) and the south, as most of the transfers are in small planes. Selous to Ruaha is about 2 hours. You can't exactly stretch your legs and wander back to the toilets during the flight....there are no toilets! So check the transfer time between the north and south and check if its for you..
Again - look at
http://www.adventurecampstz.com/
to see what a typical operator offers.
There are also a few good agents in the UK who would be worth talking to, or you can do it all yourself, probably with some advice from people on here. Depends what you want.