I've heard good things about both of these games. I've not played that many strategy games but want to try with one of these. I've seen Mandalore's video about SMAC and got interested in it but I also hear that Civ 4 BtS is really good and has a lot of mods that makes it better. I don't care about the graphics, I can adapt to the controls if they're bad and I don't mind if the game is complex as long as it's fun. Which one do you think is the better game and has the most replayability?

The answer is "yes".

Preference over which Civ is best and whether SMAC is better really comes down to personal taste. Each one approaches the series from a different angle, so it's best to try both and see what's best for you.

CIv4 goes real deep into technologies and trade, while SMAC is a lot more paranoid and combative. Both have the best diplomacy in the franchise but SMAC is regarded a level higher due to it having a narrative of its own (with great characters to boot) that isn't constrained to history.

SMAC demo: https://www.gamefront.com/games/alpha-centauri/file/alpha-centauri-demo-v1-1-windows

Civ4 demo: https://www.gamefront.com/games/civilization-4/file/sid-meier-s-civilization-iv-demo

Apples to Ananas.
Sure, they're both fruit, but they're both as different as you can get.

Civ IV is an apple. You're likely to find some variety of gameplay to enjoy it with.

SMAC is pineapple. Exotic, deep, and might even bite back at you a little.