They both scale with different stats. The swordspear generally scales with dex but it can be hard to make a build for because of the faith and strength requirements. The greataxe generally scales with strength and has a bit of dex and faith scaling. Both are good for whatever you need them for but I'd generally say that the greataxe is more useful.

I have enough faith for both I was asking cause I want to have a main boss weapon, but if you have another to recommend i'm open to propositions

What are your stats for Strength, Dex, Int, and Faith? I think both weapons are pretty good but depending on if you are strength focused or dexterity focused is what will decide what weapon you should use. If you aren't above 20ish dex or strength, you can really choose whichever weapon you want. Still I'd recommend the greataxe.

I have:

35 str (40 in the future)

20 dex

None int

30 faith