Mountain Bike Riders, Question

If you don't go to a bike shop, at least review how to fit a bike, so you make sure the bike you buy is the right size for you:

http://www.bicycling.com/maintenance/bike-fit/if-bike-fits-buy-it

https://www.rei.com/learn/expert-advice/bike-fit.html

http://www.competitivecyclist.com/Store/catalog/fitCalculatorBike.jsp

Also, in addition to the cost/weight savings of a hardtail, if you're not going off-road, it won't do much useful, and will absorb some of your pedaling energy, rather than putting it to the ground through the tire (which, in Mopar terms, will hurt your 60 foot time and your trap speed :-))