I'm sorry you're not enjoying the game but I do want to specify that Geomancer was not nerfed. 

My geomancer ended up doing several million more damage after the change. 

Hmm, my Geomancer does not do noticeably more damage than before the nerf...but he does not do noticeably less damage than before...so I wouldn't speak of a nerf as well.

From what I've gathered, it affected his ability to life steal.

Indeed, that is the con of the change. On the other hand, on the "pro" side are: 1. The reflected damage now ignores def 2. Having Giantslayer included in his passive gives the opportunity to choose Warmaster as tier 6 masterie, what is better for his A1 and A2.

It's unfortunate in the sense that it's a buff for unkillable teams and a nerf for non-unkillable, neither of which needed it, and if anything, it should've been the other way around.

I don't think his passive was inteded to work that way with lifesteal, nor did it really make sense in the first place.

Just requires different gearing for non UK teams. 

Correct.  And I agree the passive was obviously broken, blatantly obvious.  Geo works fine in both UK and non-UK, still S-Tier for CB regardless.  I think if you look at best teams in hard DT, Geo is used against some of the bosses as well.

If you could only have one of the two, what non-void epic champ would you rather have on your roster other than Geomancer?

I would choose a few others, but for an early/mid-game player Geo is definitely top 5.

He's huge for CB progression, like you said. 

There are very few I would put against Geo in that scenario. In terms of pullable champions...Deacon stands out the most there just because of his usability/versatility in pretty much every piece of content in the game. But that list is definitely short.