It depends on if you are only planning on detailing your own car, or working on a variety of paint.
It's also going to depend on how 'hard'/'soft' your paint is, how much of it there is to work on, how bad the initial defects are, and what level of 'perfection' you are planning on accepting.
I pretty much use Menzerna exclusively and most of my 2 step corrections are the combo suggested above - it's a compound (the 400) that can remove a lot of defects whilst still finishing out really well. On some paints and in some circumstances [the aforementioned level of 'perfection' required] it won't be good enough and you can then refine the finish with the 3500.
If it's only your own car, remember that once you have corrected the initial defects (for now let's assume 400 is actually required), then you should be taking good enough care of it that going forward, there should be no need to re-compound your own car, so purchase a small bottle. You should (if you are washing/drying correctly) then kinda only need to use the 3500 to maintain it.
If you are doing this on multiple cars the 400/3500 combo will do for most situations, but not all.
On harder paints and/or paint with deep defects, SHC300 munches through defects noticeably better than 400... and on softer paints and / or those with minimal defects, 3800 maybe the ticket.