they are all about the same. The number of gal filtered per day reflects the price as with adding a DI post filter. The smallest unit, usually 25-35 gal will do just fine for the tank and your house, ice maker, etc,comes with a useable 2.5-3.2 gal tank. The price is not the best, but very close, within $30 or so.
"If you think doubling your efforts can compensate for lack of skill, there is no end to what you can't do."