I'd go with 3/4 nights Selous and 4/3 Ruaha, splitting them according to your preference after you've had some time to do some reading. These two areas will give you different experiences and, I think, make for a memorable safari. Adventure Camps of Tanzania ( http://www.adventurecampstz.com/ ) has some nice camps in those areas, with specials depending on season; they are associated with Coastal Aviation so can take care of air transfer bookings as well.

We did Selous and Ruaha. Loved both for different reasons. We did two camps in each place and spent 4 nights in each. Selous Safari Camp and Jongomero were our favorites.

If I only had one week, I would spend half at Jongomero and half at Mwagusi in Ruaha. Both are great places. You would have enough time in each to enjoy the surroundings instead of packing and moving all the time.

My trip report is posted here:

http://www.fodors.com/community/africa-the-middle-east/22-days-at-4-camps-in-southern-tanzania-ruaha-and-selous.cfm

I would also agree with a 3/4 split between Ruaha and Selous, if you have a week. Where you spend the 4 nights will depend what you like- Selous has more diversity of activities with walking and boat trips, but I think Ruaha had more diversity of animals (although Selous was very good for birds). I spent 4 nights at Old Mdonya and 3 at Lake Manze- both are owned by Adventure Camps mentioned above. They also have two slightly more expensive camps- Selous Impala and Kwihala in Ruaha, and I'm going to Kwihala in June as it has been getting some excellent reviews. If you are going outside peak season, Selous Impala and Kwihala have "stay 4-pay 3" deals on so if you're going at the right time and could squeeze in 8 nights you could get 2 free nights.