I have had all of these cpu's at some time.
Speed often comes at the cost of compatibility: an 030 will run pretty much anything you want at good speed with an fpu, the 040 will run less stuff, and the 060 less still. TBH the sort of stuff 040 and 060 was made for you can do better and faster on cheap PC running winuae, unless you HAVE to have an Amiga to do it, its your money.
Getting to bench marks: if you run software optimized for it the 060 can be 4 times faster than 40 mhz 040, which is 4-8 times faster than 50 mhz 030.
If you do CPU intensive stuff that uses floating point arithmetic, then you need to have the correct libraries, the correct cpu command and a patch: cyberpatcher for 68004/60 made by phase5, 680040/60.library for GVP and oxypatcher for everyone else. There's also hsmathlibs which speed things up even more for stuff that the other patches don't patch. These patches make a HUGE difference to rendering and image processing. Some phase5 boards actuall speed up chip ram speed as well.
Memory speed does vary and can make a difference: I benchmarked my memory speed of an Apollo 1240@40 mhz in AIBB and it was faster then a published memory speed benchmark for a PPC/68040 board. And i noticed it too: booting was very fast as was copying to ram: disk, faster than my Cyberstorm 2 68060 on my A4000.
My advice: stick with an 030.