the only real measure of "powerful" when it comes to RODI, is the gallons per day it produces. Any given unit should have a GPD rating. The only thing that matters there is how long do you want to wait to make water?
For example, my unit has a pump (to help push the water through the filter, since my tap pressure isn't great and I run it from my kitchen sink), and the unit + pump is rated at 50 GPD. I haven't bothered to do the calculation to see what I'm actually getting from it, but I will say that I can get about 4 gallons of water in an hour and a half or so. In other words, it takes me a little over three hours to "fill" two 5-gallon buckets (note that I don't fill them to the rim, I don't want to spill it everywhere when I move it!). I guess doing the calculations, I get more like about 60 gallons per day, if I were to run it all day. Needless to say, when I fill up my 30 gallon brute trash can in preparation for a water change, I have to run it for most of the day.
So, unless you've got high water pressure in your home, I'd recommend getting one with a pump, as it speeds up the process. Most units are rated at either 50 GPD or 100 GPD, and I think a 50 GPD unit with a pump would be fine for you.
You'll also note that you can find 2-stage, 3-stage, and 4-stage RODI units. The 2- and 3-stage units are probably RO only (no deionization canister), so I'd recommend getting a 4-stage, 50 GPD unit with a built-in pump. My unit is a Coralife, and I've got no complaints about it. Anyways, the various stages typically include a 1- or 5-micron filter, a carbon stage, the RO membrane itself, and a deionization stage. They are all useful, to ensure that you get maximally-filtered water to use for your precious tank!