Planning a safari trip for May 2015 and interested to hear thoughts on whether Namibia or Botswana is better for game viewing at this time of the year for first time travelers. Thanks much!

My preference would be Botswana, but Botswana is probably the most expensive of the safari destination countries. There's good reason for that: the country has done an excellent job of keeping visitor volume controlled, while offering excellent facilities and experiences. If you wanted to do things other than wildlife viewing, Namibia could work. It's easy to self tour there and in addition to game viewing there's world-class sand dunes and stark landscapes to see. Still, I'd suggest Botswana over Namibia for an initial visit.

I agree with sdb2. Botswana is incredible. It would be good to know where else you are going because it is great to be able to have the diversity of landscape/habitats/animals on your trip. But without knowing anything else, Botswana!
It is definitely more expensive but if the rich wildlife is important to you and the pristine habitat then you can find suitable lodging for the budget.