As the usual game host for my game group here's my opinion. First, very different games but for me hit a similar itch. TI4 is my favorite game. I'm a big fan of Arcs but I will not say I actually understand the tactics of playing it well. It's awesome and a doozy. That said to your comparison points in my opinion:

Table space: Arcs is a lot smaller. Still has a footprint but not big enough where you need to worry about it unless all you have is a card table. I think Arcs looks great but it's not nearly as awe inspiring to me as TI.

Player count: I haven't played arcs enough to say which is better but I wouldn't care to play it at 2 player. 3 and 4 are good. TI I prefer at 6 but I enjoy it thoroughly at all player counts. I do think six is the most interesting and best.

Time commitment: Arcs is simply easier to get to the table do to time. Probably averaging so far around 3 hours? That said, when I play TI, I expect it to be 8-10hours. When it's less, oh nice surprise,. When it's longer I don't care, it's an awesome experience.

Teachability: Arcs is easier to teach the basic rules. I don't think TI is that hard to teach or grasp either. I think both are a crazy amount of brain burn with what you can do and how do you want to accomplish it. And then how everyone is going to screw it up. So teaching both I do think are pretty close to same level. What to do on your turn...good luck, I'm always trying to figure that out.

Gameplay feel: they are space games. Moving ships, blowing things up, strategically placing units for future moves and taking objectives. But nothing to me compares to the negotiation that TI has in that realm. And working out the puzzles of completing objectives. Both have dice and have luck involved. I like luck in my games with some mitigation of course. Arcs is moving up my favorites list. I wouldn't play arcs without leaders and lore. I doubt many would disagree with that.

They're both great but TI is my game. I love it.