this is purely my opinion/logic - if you have enough spindle height to use the 5mm, go with it, otherwise use the 3mm. If you're not using a clamp (which you shouldn't really need with the achromat) then spindle hight probably isn't an issue.
Also - if you go to the Funk Firm website and send them an email you may get their (more authoritative) guidance.
Also - if you go to the Funk Firm website and send them an email you may get their (more authoritative) guidance.