An upgraded A1200 with PPC is the most expensive option but it is also one of the most compatible options since it keeps the Amiga AGA chipset available and will run MorphOS or the soon-to-be released A1200 version of AOS 4.
I have a MicroA1 and it flies as long as it doesn't lock up. (It's a core voltage problem that I'm afraid to adjust from its default setting but that's another story.) The
Troka Amy '05 should be out soon and be a cost-effective AOS 4 solution.
C Source code compatability between an AOS 3.1 system and any of the others including AROS is a safe bet. Using more advanced features such as Cybergraphics4 (MOS) or Picasso96 (AOS4) will get you into trouble as far as portability is concerned. Cybergraphics 3 is emulated by P96 though so that should be safe.
As for capabilities, MorphOS is more backwards-compatible than AOS 4 but AOS 4 has a better roadmap toward the future with its interfaces in the libraries offering a clearer upgrade path to 64-bit operation while maintaining backward compatability. Although EUAE is available for both MorphOS and AOS 4 the AOS 4's built-in soon-to-be-released JIT compiler is rumored to be linkable to EUAE for good compatability and performance.
Sadly, the most cost-effective solution is a PC running EUAE under Linux or AROS (although you'd be stuck with AOS 3.9 in this configuration.) WinUAE is also better than EUAE so running Amiga software on a Windows box works well too. Also these configurations will allow you to run Mozilla Firefox which is a much better browser than anything available under MorphOS or AOS 4.
I've tried to maintain some objectivity but I think I may have let my bias toward AOS 4 slip out on this one. You asked for my opinion and I gave it.
