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

That a pretty strange build since it doesn't really have much of a focus. I will say that the Dragonslayer Greataxe will be better for you than the Swordspear. That being said, you're faith attribute is a little bit high for the weapon. Those points would be better spent on strength. That does put you in a better spot for the Swordspear but for the Swordspear you'd want to swap your strength and dexterity values.

Off the top of my head, I can't think of any other weapons that would really go well with your stats. Usually you want to focus on either physical stats (str, dex) or elemental stats (int, faith) and keep the other two relatively low. The way you're build is set up doesn't really pay off until much later when you can do a physical infusion and then buff it with a miracle.

That way you could do a heavy infusion to have the weapons scale off your strength and then use blessed weapon, lightning blade, dark weapon, or darkmoon blade to add extra damage based on your faith. Or you can read this guide I made and reallocate your attributes with rosaria.

If you do go with the first method I went ahead and made you a list a weapons that you should heavy infuse and have buffed with a miracle whenever you can. Get both strength and faith up to 40 and have dexterity at the lowest required value for the weapon with the highest required dexterity. Here are those weapons:

Lothric knight sword

gotthard twinswords

flamberge

claymore

zweihander

lothric knight greatsword

harald curved greatsword

washing pole

winged knight twinaxes

spiked mace

splitleaf greatsword

I think you got the wrong idea, I was asking for the best boss weapon for str and faith, and I planified this build a lot, I use as my main weapon heavy Lothric GS and Yhorshka's chime (both +10)

And miracles Deep protecction and Lighting Blade, with all this combine I make above 800 damage, and I'm finishing the game, my next zone is Grand Archives and I already made all the other zones (including optionals)

and at last, I told you my stats and weapons for finishing the game, with the DLC I will have both my main stats to 40, and I will use the heavy splitleaf GS

In the end, neither the Dragonslayer greataxe nor the Dragonslayer Swordspear are strength/faith weapons. The swordspear is more of a dex-focused quality/faith and the greataxe is pretty much just a strength weapon. If I remember correctly, you can't justify a single point in faith beyond 9 for the greataxe until you have 50 str and somewhere between 20 and 30 dex. Between those two weapons, Greataxe for sure but just about any quality/strength weapon with a heavy infusion and your buffs is going to fair better than either of those two.

Yeah, I understand, I wasn't searching for change my main weapon, is just in case that I want to try some other weapon just for fun which will be better for me.

Thanks, now I know what weapon I need to upgrade if I want to try it.

Sorry for the misunderstanding!