If you can do aim for the 500GB boot drive. The price difference between 250 and 500 isn’t much, but it’s nice to have the extra space even if you don’t think you’ll need it.
You do not want to be in a situation like I am… I purchased two 500GB NVMes for ~240 each. Now I need to bump them to 1TB drives. The price difference is about $100, so I’m going to spend a lot more upgrading than I would have if I brought them outright.