On a mountain bike NEVER get one of those wide seats! Period! It will just chafe your inner thighs & the 'family jewels' more. Your question brings up more questions. Do you ride on a regular basis? Are you used to going 20 - 25 miles at a time on a regular basis? If not, you have to build up to it gradually. It takes time.
You might want to look at cycling shorts with padding. If your bike came from a discount store, take the original seat off & junk it now. I use Avenir saddles (seats) on both my Hybrid bike & mountain bike. Link below from Amazon.com. Many come with free shipping.
Also, a lot depends on the placement of the seat. Many people unknowingly ride with the seat too low. They can be moved forward & backward a little. They can also tilt some. A fraction of an inch may mean the difference between pain & pleasure. See second link from Sheldon Brown. Read it all!
