Not really about "on ground", your MEL is for reference only after dispatch
you're missing the point, I suspect. Starbear's first few sentences are on the mark....
(a) if it breaks before you go, then you either get it fixed or invoke the MEL if the CFU idea appeals on the day, considering all operational circumstances .. ie, the philosophy is that the captain has the final say as to whether the MEL will/will not be invoked
(b) if it breaks after you are on the way, then
(i) you do the best you can, considering that
(ii) the MEL gives you a lot of good guts guidance in the decision making process as it considers a lot of things in which the average line pilot doesn't have any significant background
(iii) it follows that one can, indeed, go back and get it fixed after despatch if the captain considers it warranted to do so ...
Looking at the original question, the answer is straightforward .. despatch with multiple units leaves you with a back up in the event of one breaking .. despatch with one gives you the one shot only .. all a risk management and consequences consideration as embodied in the design standards and certification interpretation ...