re: Both
Both have their advantages and we need both. I don’t know why everyone always tries to make this a binary decision.
As Nasa Administrator Sean O'Keefe said during a press conference, a human in 1 day can do what a MER will take 90 days to do. You have to remember that when doing your cost analysis and comparisons.
Robots should be sent in greater numbers to more sites. However, humans can do things better. A human is much more adaptable and can perform a lot more tasks. Robots typically specialize in doing a couple things only. Humans should be sent to the most interesting sites found by the robots.
By the way, am I the only one that thinks MERs are the coolest unmanned probes ever? I commend JPL for their great work.