8.3 was my time to reset my 100 alts and finally go to 1 main who I stick with for a long time. Im finding myself swapping between my shaman and my monk depending on the day of the week and I want to just really push one instead of half progging both. I play ele/resto and ww/mw and I really enjoy both classes including the 2 specs u don’t often play.
TLDR; how do you decide who to play between classes you enjoy
I main the one I find myself playing the most and having the most fun on.
Also, theme. I love Norse and Greek mythology. Warriors have that sort of theme going for them between Bastion and the Halls of Valor.
It’s very simple. Fun. What does the word “Fun” mean to you? That’s up to you to decide.
I ask myself one question: Do I feel lucky?
No wait… I mean: Do I have fun playing it?
I loved the lore behind DK’s and the DK starting zone sold me on the class.
Pick what you like best.
I’m a fan of king Arthur and his court and the 12 knights of charlemagne, so I went paladin.
Whatever feels fun to play at the time. I’ve mained mage, enh shaman, dk, holy priest, rogue, and feral druid at various times in my wow career. I don’t do bleeding edge content so fun takes priority over anything else
Look how cute this character is!
Also never played a mage before.
Oh, thank you for this question. I’m struggling with it myself.
/e pulls out her note pad 
Druid gets around. Flight form, kitty stealth, shadowmeld to escape, Guardian mode to take on big rare elites solo, moonkin if I can do things at a distance. I do everything except healing. I don’t care for how much damage I can deal as long as I can get things done without having to rely on others to do it. Druid is the best option seeing I can do it in different ways and can always reset an encounter with Shadowmeld then try again or just get out of there.
Made my first horde warlock in outland. Love it. Then i started alliance worh a pally. Loved it. During wrath I made dk. Loved it. After wrath life changed. Had to quit. Came back during drenor and made alliance warlock. Since then I been playing my lock. No regrets. Love my warlock.
My main is the character I have completed the most achievements on and the character that I achievement hunt with.
Prior to MoP I mained my draenei hunter.
I have a deep love for hunters, hence why I have something like 6 or 7 of them, all with their own themes, at various levels.
Once MoP came out however I made this pandaren monk and he was SO much fun to play, rolling around, rocking through mobs, it was a blast. From that point on I mained him for ally and never looked back. He does the vast majority of achievs, mount farming, gear farming and the story. I don’t do pvp and rarely touch current dungeons or raids, but if I were to do any of that, he would be my man…uh…panda.
As for horde my main was my belf rogue up through most of Legion, that artifact weapon buff to NOT die from any height was fantastic. At some point it shifted to my belf DH though and with the parkour style of double jumps, the agility and the wings, he has been a blast to play, especially in places like Suramar City or high cliffs.
I doubt I was very helpful, but I play both factions, and have a lot of alts, so I’ve tried all of the classes and races, but these still stand out as my favorites to play right now, even if I do sometimes avoid them for weeks at a time to level others. 
My main was typically my druid, since you can be everything with one class.
I don’t choose. I always have the same one.
For me its 2 questions:
Do i like the feel of the rotation and speed?
Can i perform in a decent lvl with it?
If any of these fail, i drop the spec.
I play what I enjoy the most. If you plan on doing very high end content, maybe take into consideration the viability of your class. Otherwise, play the class you look forward to the most when you log in.
I just main something I have always played a lot and it depends on the situation of the class when I start playing again, almost always dk or ele sham.
Between those two classes you listed, what do you prefer, ranged or melee? The off spec healing is covered on both so deciding what play style you like more will help nudge you in the right direction. The only thing I would consider is if I decide I want to try tanking or my group needs a tank at times, then the monk will be the choice to cover anything that may be asked.
I usually have a goal for each of my characters and\or an over all goal I want to accomplish in the game. I have limited time and am in no rush what so ever, so I do what I want to do. Even it it takes the entire expansion or into the next, so what… So no matter what I’m doing to get some entertainment it is actually accomplishing something.
Sometimes I do things just because. (well most of the time really)