if you are using your amiga for mostly CPU intensive applications (large spreadsheets, 3D modelling etc) then go with the 040 over the 030 but if you are just using it for games , especially with WHDLoad stick with the 030 as in my experience it's the most compatible with WHDLoad.
I had an 060 in my 1200D for years but then converted it to strictly a WHDLoad machine and had issue after issue...put in my 030@50 and all issues gone
