Second Army - thousand sons or demons?

As per the title really. I'm thinking of selling up my ad mech and starting a small second army. Either thousand sons or chaos demons. I do like the tzeentch stuff and a demons army would be quite heavily tilted that way.
Do thousand sons offer me a better tzeentch option than a pure demons list? Are thousand sons quite viable with demon heavy lists rather than using a lot of power armour? This is all looking for a fun second army, rather than a top tier tournament army. Thanks!

So I have a thousand sons friend and when he talks about codex thousand sons it should be replaced with codex Tzaangor. Because the majority of units you'll be running will be tzaangor stuff if you are playing the army even somewhat competitively. Most marine stuff is inefficient unfortunatley. But on the other hand he adores the army and has a blast playing them

I don't know how Tzeentch daemons are standalone but I do know that they mesh pretty well with thousand sons and you can get some very nice stacking buffs on them so you don't REALLY have to choose which one to play, just which to grab first.

That's not so bad. Tzaangors, Vortex beasts, chaos spawn and the demons are one of the main things drawing me to tzeentch. I prefer the idea of sorcerers and beasties than going power armour and tanks. It makes them feel different to other marine armies. Thanks for the info!