Outside of the Wizard’s saves, the consideration of the spellbook (as some DM’s may have restrictions on buying your book as you become a wizard), and the character’s history and background, there is no real advantage to choosing Wizard at first level if multiclassing as a fighter.
The fighter would lose heavy armor proficiency if multiclassed in to, and a high Con save has serious advantages, as you pointed out. The weapon proficiencies are a nice addition depending on character concept, but that is something you gain regardless of whether you take it at 1st level or not.
As for the cleric, they gain the armor proficiency regardless of what level if you choose an appropriate domain, they gain spells, and they promote normal spell progression. Overall, mechanically, I would say that Cleric is the stronger option. They aren’t likely to do as much damage with weapons, but access to Cure Wounds and similar spells that are normally barred from Wizards is certainly a huge bonus.
If cleric is chosen, the choice of which class to take first isn’t nearly as clear, nor is it as important.
Ultimately, look at the class features of fighter and cleric to decide which one you think is better for your goals. If you choose Fighter, it is advantageous to take it at first level. If you choose Cleric, it isn’t really important which you take first.