Saw that you went with Finn. Can't go wrong with either choice.
But my 2 cents, having just gotten my zFinn, and have had the CLS zeta for IBaT for a while. My take is that zFinn turned several unused characters into another team that I can play with and can depend on for GW, TB, & Raids. It was far and away the biggest improvement that a single zeta could give me.
However, I am still glad that I did CLS first, because I was still completing GW/TB/etc without Resistance. Competition on my PvP shard is fierce, and pretty much anyone in the top 50 could be top 5 at any given time. We're not all whales or anything, just that a lot of us are very evenly matched. IBaT made an immediate and noticeable improvement in my arena team. Easier to advance, lost fewer spots on defense. Not nearly as dramatic an improvement as zFinn, but just as valuable to me, in the situation I was in.
My suggestion would have been to base it on where you feel you need the boost NOW. If you're teetering on the edge of a boost in Arena, and are managing TB/Raids/GW without Resistance (even if it's not ideal), then I'd say CLS. If it can move you up in PvP and improve your rewards, it will pay off better over time. If you are hurting on those other things, need another team to complete stuff, etc., and/or are very comfortable with your arena ranking, go zFinn.