A 75gpd unit would be good. You also want the ability to make a decent amount of water quickly in case of an emergency. That is another great reason to have one as well as the water quality of course.
If you have a 150g, then you will want maybe 30g for water changes a week, and another maybe 10g a week for evaporation and top off.
I run a 150gpd unit for all my tanks and make about 200g a water a week. But there have been times i needed to do a 50% water change on a tank ASAP for one reason or another and I can make what I need quickly.
In my opinion, if you anticipate being in the hobby for an extended period of time I would get the best gpd unit with the least amount rejection. Means you get more water and less waste water. Bulk reef supply sells top of the line units for less than $200. I paid 180 for the deluxe 5 stage I believe, during a group buy on this forum. It has 2 carbon stages, the sediment stage, DI
and of course the membrane. It also has gauges for tds
of the water going into the system and water coming out, to see when you need to change your filters.