I think Amiga / MOS is likely to be quite different in responsiveness, that is they will "feel" fast even on relatively slow hardware (only BeOS is similar in this respect), unfortunately that's all that's ever to likely to appear.
With a dwindling user base the hardware is actually getting slower with time, the Pagasos II may be 5 years old but it's probably quite a bit faster than the Sam!
However it is the only system that will run MOS and OS4 if you want to try both.
As for reliability OS 4 and MOS are based on the Amiga OS design that was built back in the 80s, without memory protection a misbehaving app is quite likely to bring down the entire system. That said Win 9.x supposedly has memory protection and should be pretty stable but I found Amiga OS 3.1 to be a lot more reliable.
OS4 appear to be experiencing some issues with the hardware that haven't been quite worked out yet, that appears to currently causing some reliability concerns.
Another option is to wait and get MOS for the (PPC) Macmini. If it ever appears, at least the hardware will be cheap and good.
