Invoker concundrum

Invoker most badass hero in dota hands down, I'm still at the point where I'm noob with him, and more comfortable with Quas Exort, played it more, easier to last hit for me atm. Practicing Quas Wex a lot but it's apparent there are instances where you'd want to choose one over the other. I've fallen hard in ranked jumping right back in after a long hiatus, and watching a replay from last night, against a tanky lineup, I felt in hindsight going 4-0-4-1 would've been more beneficial to the team. I didn't do too hot, and just didn't have the date to help out when it was needed (night stalker kept suiciding for kills and sniper fed a lot) but I could've done more with damage:
http://dotabuff.com/matches/615054638

This is an older match where changing the build to max exort made a huge difference:
http://dotabuff.com/matches/473177258

In what instances would Quas Exort be superior to Quas Wex and vice versa?

It mostly depends on your lineup, if your team needs more damage quas exort is the way to go. It is also better at splitpushing and farming up.

But quas wex give your team great controll and counter initiation.

Exort gives better pushing power and much better single target damage. Sunstrike is useful if you've got something like a Clockwerk or long disable and want to be able to contribute without leaving your lane. QE Voker is also about as close to autowin as it gets in terms of mid matchups (OD being the only one that isn't completely destroyed)

Wex gives Tornado+ EMP, which is (no exaggeration) possibly the best single-hero teamfight combo in the game right now.

Exort for gank and push, Wex for teamfight. It really depends on your ally picks, but in most games I find Wex is more useful.

Saw a match today where the Sunstrike was constantly and successfully chained onto a Centaur Warruner Stomp.

If what you said about the match is true; then it wasn't really your fault. Hard to win when two of your guys keep on dying all the time. Also not fond of the four core line up. Your decision to go Quas Wex would be right due to the high need of CC.

Your Quas Exort build earlier was good cause your team already had CC through shaman and Jakiro.

Dk vs IG when Iceiceice and Mushi were crushing IG, was awesome, awesome indeed

Quas- Wex --> Initiation and mana burn
Quas- Exort --> Pushing power and global kill secure (for aggressive tri/dual lanes)
Really, it's that easy.

I usually go QW because the mana burn is kinda overpowered at the moment, plus I like the extra movement speed and early presence: with QE you might get some kills with the Sun Strike, but with QW you can go ganking and win a lane. QE Invoker has those Forge Spirits that work wonders with Lycanthrope, another top tier Hero, but versus a tanky lineup I'd probably go QW anyway unless they have no counterpush or you can really abuse that Sun Strike. You can still max Exort second with the QW build if you feel the Chaos Meteor is needed.

I think this description and watching replays in conjunction, I was sitting mid with QW not moving until it was too late. I played it like QE, farming mid..minus sun strike. I'll work on it, I just need to get the timing down for EMP, I blew it a few times.

Tornado + EMP

The timing changes depending on your level of Quas and Wex, so it's not exactly straightforward.