Upgrading a heavy duty bike
Well, going SRAM means new shifters and new derailleurs (and while you're at it, you may as well install all new cable housings and cables). Depending on your hub, you may need to get a new hub (or more likely, a new rear wheel) to fit a 9/10 speed cassette that the current x7 group uses -- however if you have an 8/9/10 speed freehub already, you should be good to go. Note that the # of gears in the front &amp; back have to match what your shifter expects.

As for brakes/brake levers, depending on what levers you have, you may need new ones (if they're in the same unit as the old shifters, for example, you need new ones). You're free to mix and match the brands of brake levers with the shifters, but you may find that the proprietary clamp systems (e.g. Matchmaker/i-spec) will give you better positioning of the shift and brake levers than mixing and matching. 

I don't think SRAM or Shimano will be "easier to work with" -- go with whichever one you prefer (usually dependent on brand loyalty/how you prefer to shift).