Build a "fat bike" like the Surly Pugsley or a regular front suspension MTB? Budget will be ~$700. I assume I'd probably just buy the regular MTB or build the fat bike from parts I acquire.

This will be my first time really getting into MTBing so it's tough for me to say with experience, but just assume my terrain consists of typical mid-western stuff, I think. I don't Think we have anything crazy or super rocky around here. I know a front suspension bike would be faster on downhill sections of singletrack, but I'm mostly just looking for fun factor, fast or slow. A fat bike seems like it would be able to tackle more, but at a slower pace. For those who have tried both style bikes, what did you find more fun to ride on various terrain?