Linde is one of the best blue mages in the game only maybe outclassed by reinhardt / on par with delthea, furthermore your linde is pretty optimal assuming she isn't - attack, there is absolutely no reason to use mae over her because she outclasses mae in the most important stat which is speed and outclasses most other units, unless you want another blue mage for like arena assault or something. Even then there are better options out there like the aforementioned Reinhardt and Delthea.
+Res -hp Mae
Just pulled a 4 star +res -hp Mae. Compare to my other 2 Maes this one has the most ideal Ivs. The thing is have a +speed linde as well. Is this Mae worth investing and maybe making her 5 stars?
What your Mae has going for her is basically being a duelist. Otherwise, at 5*, she'll have only 31 Spd, unless you want to use Fury, which will bolster her magic soak and attack.
As much as I adore Fae, I would not recommend her unless she is +Spd as it is her greatest investment that will increase her survival in the long run. She is great as a Life and Death candidate and probably still has more resistance Linde, if only by a little bit.
Though, she is way more common than Linde in summons now, and has access to be more merged by quiet a bit. I have a 5* +5 Mae who still has her Owl Tome, and she performs magic soak quiet well, even unmerged. She can be a great tank if using her Owl Tome, but it's hard to get it's full affects.