Easy 68010 question...

[psxphill]

move.l (A0),D0 ; Speed zooms on 030 but 020 has no datacache so it has to sit around all day waiting for the RAM to cough up the data.
 
Those sorts of instructions happen all the time.

Only when the data has already been cached though, otherwise it's just as slow. It's only got 256 bytes of data cache & most software will have a larger working set than that.
 
There is no doubt that a 68030 is better than a 68020, but in an a500 it might be a bit of a waste. I had a 28mhz 68000 accelerator for the a500 and that was pretty good.

[psxphill]

Along with the datacache, the full 68030 also has an MMU on the die, which no 68020 has.

No, but it's 68851 compatible. So a 68020 + 68851 is equivalent. The 68030 works out quicker when using an MMU though because of that. The 68030 also supports a faster bus than the 68020 and it can be clocked faster.
 
All these variables make the 68020 vs 68030 comparison much more complicated than saying the 68030 is better than a 68020.
 
The quickest 68030 board could make a huge difference over the quickest 68020 board. But at what cost? A bad 68030 design compared to a good 68020 design might make it hard to justify spending more on a 68030. So unless you have two boards in front of you and can quantify the difference in performance then it's a pointless discussion.
 
Personal circumstances also play a part. Will you use the MMU & will the software you want to run benefit from the data cache.