What yo are giving up between eh 2 models is NOT internal space...But, the 2 Star is ALL stainless. The 1 Star is a very corrosion resistant sheetmetal, just not stainless. And - without eh 2 Star - you have the warming box that also gives you a side shelf/table top to hold things.
You will not see a difference in performance. The accessories are always something you can add as you go - but I like your idea of having the upper 1/2 grate You may be able to put a larger, or full grate in it as well. I have 2 - 2 Stars, and I really like that feature for loading the grill up and doing a big cook in a compact space