Best entry level heavy rider bike: Northrock XC00 vs. Mongoose Mushaboom
You have chosen these two bikes based on the information that you know.
Experience riding either of these two bikes is not listed as part of your evaluation criteria.

Deciding which bike to buy is: 


Very important because:
A. It is a large purchase
B. Your choice will have a large effect on how much you enjoy riding. If you get a bike that is not fun to ride you won't want to ride it and it will sit in the garage collecting dust.
Very difficult because:
A. There are so many bikes to choose from sold by several different vendors
B. There are so many opinions
C. A good choice will be made based on many factors - some of which you can't know without knowing your own preferences.


Due to the reasons listed above don't buy a bike without riding it first.

This means that you need to visit your local bike shop(s).  

A good shop will


Give you friendly no purchase required advice.
Let you test ride bikes to see what works for you.
Be more interested in you getting the right bike than making a sale.


After visiting the bike shop you may have narrowed your search down enough that you can find a place that will rent you the kind(s) of bike you are interested in. This will get you some real saddle time so you can learn what your preferences are and factor them into your buying decision.

Other information from Bicycle Stack Exchange that might be helpful when visiting the bike shop:
How to choose a bike shop
What do I get when I pay more for a bike?
Choosing my first bike