The 30 would be find, I don't know how much room you have, but I supose bigger is always better
. In many cases, depending on how you make your sump, you can use the sump as a refuge. If you do use the 50, which means much more water evaporation and topping off, you will have plenty of room to use it for both. The 50 gallon will be adding a lot of options for you and increasing your water volume by a lot. Which also means you have to add more meds if anything should happen, more ferts if you have plants, I'm sure you've taken all of that into account though.