All these variables make the 68020 vs 68030 comparison much more complicated than saying the 68030 is better than a 68020.
That was sort of my point, really. It's not a trivial comparison when you are judging from a price/performance ratio. A full 030 is clearly faster than a 68020 + external MMU, but if you don't need the latter, then clock for clock, the 030 isn't much faster except in those cases where the current working set can be effectively cached. That's tricky to gauge, because the 68030's data cache is minuscule.
Personal circumstances also play a part. Will you use the MMU & will the software you want to run benefit from the data cache.
To put everything in perspective, if you are going from a vanilla 7MHz 68000 to, for example, a 14MHz 020 with Fast RAM, the difference is going to be readily noticeable and only gets better as you scale up the clock.
Of course, if you can find one that works and can afford the likely stratospheric price, absolutely nothing will trump the Progressive Perhipherals PP040-500/33MHz for raw A500 acceleration. Shame the RAM can't be expanded, but you can't have everything
