Wanting to create a shifter Cleric of Selune or Sehanine Moonbow that focuses more on the casting side of things, with a Domain like Knowledge, Grave, or Light. Was wondering what shifter type might be best for this, especially in a game with flanking rules. Beasthide seems too front-line oriented, and longtooth just is useless when you're dumping Str. So I suppose it's mainly down to Swiftstride and Wildhunt; whichever of these ones might be most useful?

Or would it be better to just go with a race with access to flight lol? Cuz aasimar and owlin were next on my list of races to try.

I love the idea of a shifter cleric of a moon diety, great flavor.

If it were me I would talk to my gm about letting me do a homebrew variation of a swiftstride shifter that got a fly speed equal to walking speed for 1 minute instead of enhanced walking speed. 

It would be similar in power level (actually a little less, but that's good for convincing the gm that is balanced) to a protector aasimar's radiant soul ability. 

I'd go with Swifstride. Especially if you want to prioritize being a spellcaster. Being able to cancel advantage against you is useful, but I think you'll get more value out of being able to strategically move further each turn with the increased walk distance, and the ability to get a free disengage from enemies when it isn't even your turn.

If you just want flight, I'd take Owlin over Aasimar... if only because an Owlin can always fly, while the Aasimar can only fly once per day. They get some other cool features, but I think always-on flight is more impactful.