I went for the bigger capacity machine - saves you filling up more often. Possible to do a whole house with the bigger prochem. There is a downside however - its heavy, but then I am old! 75ft of hose means you can leave machine at front door though. Have managed without a heater as I mainly do domestics and can always access hot water. Even with cold water I have managed good results. There is another thread on here suggesting inline heaters on prochem machines are a waste of time. If you are on a tight budget this machine can be bought much cheaper than the competition, but I still get great results and am very pleased with it...
...but probably only until someone gives me truckmount to try!