Between the two, which would be the better pick? Wanting an rdps character that can have a healing offspec.
Probably the Priest, both are fine as healers, ele feels clunky, shadow feels slightly better. For rdps neither compares to hunters, mages or warlocks imho.
Yeah, but none of those have a healing spec.
The reroll spec 
Which is?
/10 chars
I recommend making a trial character for both and see which you enjoy more.
I have both at 60, so this doesn’t help.
Elemental is in a really good spot right now for things like PvP and raiding, but is bad for M+. Priest is fine in all forms of content, but is countered by Ele. If you have no desire to PvP, I’d do the priest. If you have any intention on PvP’ing, I would absolutely Ele.
Edit: I say this as a priest main, for the record. Shadow is a lot of fun, but in PvP, resto and elemental shaman are beyond good.
Then go play them and do heroics and see what you like better?
On one hand Shadow is doing well right now. How long that will last is anyone guess. Priest has two healing specs, holy and disc. I don’t know much about disc never cared for it but Holy has different stats priority depending on what you want to focus on M+ or Raid.
Shaman has an interesting turn this expansion with each of its specs doing okay enough to not get the ol “should just reroll” call out. Not that resto has had that issue often. They seem fine, dang near impossible to oom one and Ele looks like a lot of fun.
Though if you pick ele shaman I’d go do the World boss on that character for one of its bests legendary memory to drop. Or wait another 4 weeks.
Go with preference + enjoyment. Range DPS in general is a good idea so you can’t go wrong.
What would I pick?
Elemental shaman. Decent class, great healing spec, ranged damage.
Just pick what you like. Spriest is probably slightly better than ele shaman, but rshaman is a bit better than disc/holy.
The difference between them wont really affect you performance-wise so again, pick what you enjoy.
If that’s the case, it sounds like you want to pick fotm without actually saying you’re looking for fotm.
I guess priest for now? 
Shadow’s great and all but your other two spec options are both healing specs. If you like the idea of being able to swap between ranged, melee, and healing. Shaman is the choice.
If i liked elemental or enhancement enough right now i’d have mained shaman this expansion, but i didn’t so i’m maining priest. Holy and disc aren’t my favorite healers, but i do love all of the tools holy has at its disposal. Holy Word: Salvation and Divine Hymn are my favorite raid cooldowns.