Slitherine

When you have to decide, is it better strategy to have a weaker unit with more experience or a stronger unit with less experience?

I think you are asking about not taking a replacement vs taking a regular replacement in the middle of a scenario. If the unit can survive at the weaker strength, it's better to keep the experience and the prestige and do replacements at next deployment when they are cheaper. If the unit doesn't survive, of course, that's 0 xp, but also 0 prestige spent, so depends which is more valuable to you at that point.