The only reason I can see not to pick Rubio is he might overshadow/outshine Romney. As the last election showed, there is a different percieved gravitas between a first-term Senator and first term governor. Rubio gets a leg up over Martinez there. Don't know how she presents on camera and public speaking, but Rubio does so quite well indeed. He would likely pass the "voter comfort test".
Ironically, Palin's dismal performance may help him there. He'll initially be compared to Palin (statewide officeholder for only 2 years before tapped for Veep), and Rubio is at the opposite end of the spectrum in poise and speaking ability. The comparisons would only help elevate him.
Martinez seems extremely authentic. She could look women in the eye and connect. She's no Sarah Palin. And I don't think she's a risk, either.
http://www.youtube.com/watch?v=Ek3OSIQVYQE
You gotta admit--she's good. She's my choice for the 2020 Republican primaries.
She genuinely doesn't seem to want it though.