Question about Colossal swords: Zweihander vs. Greatsword (GUTS)

I started a strength build, and am about level 35 or 40, with about 35 in vigor and starting to level up strength. I started out with the Zwei because you can just buy it at low level for cheap, and two hand it. I noticed it was fun, but I was having more success with other starting weapons, like the Lordsworn greatsword. I leveled it up anyway, to the max I can atm at +12. I also found the brickhammer, and a partisan dropped. Both seem to be way better at this level. Then I grabbed the Greatsword, and holy cow, it beats the hell our of the Zwei. Then I noticed the scaling is different, and the Zwei scales better with dex than str (I forget the letters, but I think it is significant, like not that great with str).

I wonder if this is the issue, because my starting class is the str guy, and that is what I am leveling - although it is still really low, like maybe in the 20's - I can't even one hand any of these str or colossal weapons. But when I 2H the Greatsword, holy cow, the AR is like 401, which is like 50 - 100 more than any of the other big str ones that I have found so far, and also much higher than the partisan.

I am having the best luck with the partisan, because the moveset is so much faster, and I can 1H + shield it, whiich is very handy for NPC's. But I think the GUTS will be better for bosses at this level, idk.

The other thing is, the zwei puts me into heavy load with anything else besides a partial armor set equipped. The Greatsword puts me into heavy load even worse, in fact I think I am going to have to go without armor if I want to be in medium, until I level up endurance, which I was hoping to avoid for a bit. Guess I will farm Grael to level up a bit.

I don't want to level up dex for the zwei, because I am committed to a str build for this char (this is my first playthrough). Is the zwei any good without leveling dex? I am thinking not, but I was so surprised to find out has better scaling with dex and not very good with str. Seems you would need a dex or dex/str char to optimize it? I don't really know though, because I am just learning the game at this point.