There is usually nothing wrong with most of the small under the counter units sold by Home Depot and Lowes etc accept that they are small capacity units. In some respects they are better as a RO unit produces higher quality water at a more effiecient rate when run for a long period versus many short periods of time. However some of the small units have very small prefilters (sediment and Carbon) and those would need to be changed quite often when producing the volumes water used by most reef tanks. Avoid units that have screw on prefilters. Buy units with housings that hold replaceable cartridges. With small tanks the small Home Depot RO filters might even work best for a lot of people. However, as long as the filters elements are at least 10 inches long they are considered full size prefilters.
The efficiency ratings and the rejection ratings are usually the same for the small units just the volumes produced are lower. Reef Central has several Sponsors listed by link from the Home page and they all can provide larger RO, RODI or DI filters at good prices. I would recommend that you use two DI filters inline following your RO filter.
IMO Stay away from SpectraPure unless you like more than you need at much higher prices than justifiable. Their filters are like paying for a Ferrari,receiving only a Mercedes when only needing a Chevy to drive a block to buy a news paper. All the other Sponsors (ie The Filter Guys, Bulk Reef Supply etc.) will sell you comparable products much cheaper, but would more likely just discourage such purchases of unneeded high end products..