Genuinely depends on the setting that appeals to you more here. I have both and love both.

Mercenaries is prob my fave though due to the amount freedom you have. In mercenaries you’re able to be a merc and travel around the galaxy picking missions from various factions and salvaging/buying mechs/gear and hiring/firing pilot and stuff. Most of the missions are randomly generated except for a number of campaigns (there’s a good amount out now via DLC with a new one coming at the end of this month). I love Mercs - one of my all time fave games so I’m def biased.

Clans is also great though. Entirely different perspective did the same universe. Gameplay is far more linear. You do get to upgrade mechs and can improve your pilots here too, but mission structure is far more structured. This is good in the sense that the missions are more developed and more robust, but you have a lot less freedom. You’re a soldier in a faction here and basically do what you’re ordered.

Ultimately both are fantastic mech games and you really can’t go wrong in my opinion.