I will disagree with Tony on his statement that all RO/DI units are pretty much the same. While outwardly they may appear the same its what the filter housings contain that make all the difference in the world.
Quality prefilters and carbon blocks are there to protect the more expensive RO membrane which does 90-98% of the work and which in turn protects the DI resin or cartridge.
Many vendors tout the word "Stages" like it really means something when in fact its not a matter of how many "stages" but what those stages contain. I can make a 3 stage system outperform a 7 "stage" system any day of the week. When shopping for a RO or RO/DI system you want 3 stages for RO only and 4 stages for RO/DI. If you havs special conditions like high sediment loads or high dissolved CO2 you might need another special stage to deal with those but its rarely needed.
Look for a system with the lowest micron rated prefilter you can afford, some go as low as 0.2 microns such as the one I own, others use a 10 micron prefilter which is slightly more effective than a screen door when it comes to protecting the more expensive carbon block which is its purpose. Two carbon blocks are not necessarily better than one, in fact two can be detrimental as it often causes increased pressure drop leading to lower GPD production and lower finsihed water quality. A single carbon block in a 1 micron or less range, 0.5 or 0.6 microns is best, is all you need for chlorine or chloramines or VOCs.
You will want at least a 75 GPD RO membrane and in the case of Dow Filmtec ypu do not want the 100GPD as it is not a RO membrane at all and is only 90% efficient versus 96-98% for their 75 GPD. In the case of Spectrapure they start with a 75 GPD Dow Filmtec then treat it with a special process then test its perfromance, either individually or in batches then they are shipped wet in a special solution to increase perfromance and lifespan. Other vendors use dry untested off teh shelf RO membranes, remember I said they may outwardly look the same but looks can be deceiving, its what is inside that counts.
You will want a ful sized 10" vertical canister type DI filter with a refillable 20 oz cartridge inside which is far superior to the small horizontal type no matter how many they stack on and call "stages". The verticals fill from the bottom up so all DI resin comes into contact with the treated water while horizontals short circuit or channel causing spotty treatment. All the vendors Tony mentioned offer vertical DI filters.
You will need a TDS meter to determine how well the system is working and when to replace the RO membrane and DI resin, a pressure gauge is also a good investment as it can tell you a lot about filter condition and system performance.
I can go on and on but there are literally hundreds of threads on RO/DI on this and many other reef forums.
A few places I would suggest looking are:
Look at the CSP-DI
75 GPD premium
Optima Automated is a good choice
Marc uses GE RO membranes so the 100GPD thing does not apply, compare the 100 GPD GE to 75 GPD Dow Filmtec, they are almost identical in every way.