Hello,
Looking for some input from more experienced players - have been playing for about a week now and my best characters are right now Juggy (lvl 40), Black Widow (32… yeah I know… a mistake… no scaling skills) and Mark 35 Iron Man (lvl 50).
Now my Thor is 30 and 3/1/2 - I got another red cover and am actually quite hesitant to use it since I wanted to get a 3/5/5 specc…
Should I level him anyway or what? 
If you are wanting to build around Thor, I would wait. IMO, 3/5/5 is a way more functional build than 4/4/5. Those extra 2 green tiles are huge when trying to get free cascades/extra turns not to mention feeding the storm. Pair that with the desert level, and you will be very strong! Also, I feel you can get a pretty good amount of Thor covers from prologue, events, and random deck pulls, so it would be well worth the wait to me.
FWIW, I’m in a similar struggle as I have an OBW that is 3/5/2. I don’t ever see black covers, but I am holding out because I have a similar view on her. In that span I’ve passed on about 3 or 4 purple covers… it can be frustrating!
So the thing about Thor is that once you make it past 2* land into 3* land, his only relevant ability is the yellow due to it being an AP battery and there being a lack of good yellow skills in the game: red/green just end up not really being used. However, the green is useful while you are in 2* land, so i guess I would hold off on the red. The downside is that your Thor doesn’t get more levels, but thats probably okay.