Think of a Pegasos + MOS as an easy way to reach the fully expanded classic Amiga level, and then some.
You will essentially skip the hassle of buying expensive individual parts and OS3.x patches/hacks which are needed to make the workbench look nice and fancy and expand the classics to the up to date (or so) level, and you will gain more stability in the end, and of course run everything through a PPC Amiga like OS...much, much faster.
What you will lose is compatibility with classic hardware banging games, but for that you may as well keep your A1200.
The same could be said for OS4.0, but if you can source a used AOne system.
