I guess it depends on how much water you plan to run through it per month. I myself would not be satisified with 15 coming out of the membrane. Ideally it should be 6 or less. At 15, long term, your DI costs are more than double.
I will never use horizontal DI filters. Standard 10" Vertical is all I will ever use even better with the dual. On your next refill, I'd spend the little extra to convert that.
Looks like I may be picking up a 75 gallon unit from BRS. The DI cartridge on this unit is not refillable meaning I'll never be able to expect better performance on this. So I can either add a seperate DI unit that I can mount vertically and refill, or just keep it simpler and get a 75 gallon unit.
I am now getting TDS 0 water. I picked up a single canister DI from BRS. Remounted both my Mighty Mite and the new single canister DI. Now I am getting 0 TDS. I also mounted it more conveniently. This is in my basement bathroom (the guy bathroom) so I ran a longer length off the DI and just fill my bucket right in the shower. Since I haven't sprung for an auto shutoff yet, if I forget to turn it off at least the water will be going down the drain and not across the floor! Oh..and kbb0118...65 lbs of pressure going into the unit.
I think I'll run this way until the membrane starts to go then I'll switch out to a 4 stage RO/DI + my single DI canister I'll have a 5 stage. That seemed to be the best solution to get the most value out of the system I have while setting myself up for a better system in the future.
Membrane is going to last 4+ years. With DI costing about $8-12 a refill, getting higher rejection membrane sooner rather than later will save you more than the $45 for a new membrane over 4 years. The membrane cannister you have is fine and univeral, no need to buy a whole new unit just get the membrane and new flow restricter to match.
Also, get that shutoff ASAP. That's money wasted in an overflow. When I lived in an apartment and didn't have it hard plumbed, I probably went to bed and forgot it running more times than I want to count.
I would do both of those items before spending a dime on any new livestock.