I can’t speak to math at Grinnell, all I know is that it is very difficult.
As for econ, it depends on what you want to do. We are very strong for going into academia, but we have less connections in finance. Of course, I still know some motivated Grinnellians who have successfully gone into finance. It’s less a part of Grinnell culture to seek wealth rather than ideals. There is a Student Endowment Investment Group that you might want to check out if you are interested in finance:
http://web.grinnell.edu/groups/invest/
Undergraduate research opportunities are plentiful for those who take initiative. All you have to do is show genuine interest in a professor’s research and ask if you can help out.
There is actually an economics professor (Caleb Stroup) who just left Grinnell for Davidson College. You might want to email him because he would probably know the most about how the two colleges compare. If you do, make sure to be very professional and ask specific questions.
What I do know about Davidson is that the campus culture is very different. I find it difficult to imagine that a happy Grinnell student would enjoy Davidson’s culture. The same goes for happy Davidson students. Do your research on campus culture beforehand because it will make a significant difference for your future happiness.