I use a 7 stage filter from The Filter Guys. 3 filters + a DI Filter and 2 membranes. Makes it a bit more efficient (as the waste water is ran through over the second membrane so you get a better ration of RO/DI water:Waste water.
You can always add on a DI cartridge to any system. Are you 2 filters + a DI cartridge, or 3 filters, no DI cartridge?
Anyway, it really depends on your needs (volume water, etc.) and the quality of the water going in.
Here are water is very clean, although there are spikes of contaminants maybe a few times a year. So, heavy filtration isn't really needed