I don't really see why Son being better than him is a reason not to sign him to be honest. Sanchez wasn't better than Toby or Jan when we signed him but he's been a huge success. Just so you're happy, I'll answer your question though. Right now, Son is a better player than Martial. However, Son is missing a number of games this season (Asia Games in August and Asia Cup in January) and could miss a whole 2 years due to military service. Martial is someone I really rate and is only 22, 4 years younger than Son. I can't really answer whether Lucas is better yet because he hasn't been given the chance in the PL.
Just like Sanchez last Summer, we aren't desperate for Martial right now but who knows what will happen in the future. The fact that we are looking like moving to 4-3-3, and you could argue we only have 3 out and out wingers in the squad suggests we might need another. Especially when one of them is N'Koudou who I think is poor. I also think Martial is more suited to playing as a backup to Kane at striker than Son is.