If you can afford a few hundred more dollars and have enough space, go with the HP-60 or 100. Ive used a Mytee Lite II for years and while it works GREAT, there have been times that I wished I had larger tanks and more water lift.
Mytee Lite II=3 gallon recovery/solution tanks. 85" water lift
HP-60= 6 gallon recovery/solution tanks. 137" water lift
Ive detailed A LOT of RVs and always have to stop and empty my recovery tank and fill my solution tank a few times for each job. This takes a lot of time. Its also really annoying when the recovery tank starts to get full and the water lift starts to diminish, leaking water from the hose back onto the carpet.
If youre just extracting the interior of one car, truck, or SUV you shouldnt have a problem getting through it on one tank with the Mytee Lite II. Just make sure to empty the recovery tank before it gets too full and wipe away any lint, hair, and carpet fuzz from the screen to ensure you dont lose any lift. DONT over saturate the carpet!
The Mytee Lite II is a great unit for the money and works very well for your everyday detailing needs. Its also very well built and lasts a long time. Ive used mine DAILY for almost 3 years with no problems (knock on wood).
On a side note, the only concern I woud have with one of the larger units is WEIGHT. The Mytee Lite II is pretty heavy when filled with solution so a unit twice its size would be pretty hard for one person to lift in and out of a truck/van or into an RV to detail.