I know it's been said that you would be much much much better off with Great Axe than Great Sword, but out of curiousity, why do you want to use Great Sword on Warrior? If it's just a matter of aesthetics, then you would definitely be much better off using Great Axe to get Warrior to 30, then unlock Dark Knight. I know it doesn't seem like much of a difference at this point, and to be honest it really isn't that much... at level 18. I know it's something you really probably don't want to hear, but the difference really is pretty considerable.
I'll break it down for you.
At level 18 a Warrior has a cap of 57 for Great Axe and 54 for Great Sword, which is a difference of 3 Accuracy and 3 Attack.
At level 30 a Warrior has a cap of 93 for Great Axe and 89 for Great Sword, which is a difference of 4 Accuracy and 4 Attack.
At level 37 a Warrior has a cap of 114 for Great Axe and 109 for Great Sword, which is a difference of 5 Accuracy and 5 Attack.
So you can see that the gap gets progressively wider in terms of skill, but there's more to the two weapons than just skill difference. Let's look at axes.
At level 18 you would be using the Moth Axe, probably, which has a base damage of 32, a delay of 480 and an extra point of DEX as a perk. Your next axe should be the Neckchopper, damage 39, delay 480, Accuracy +5. For Great Swords you say you want to use the Braveheart, which is damage 30, delay 444, Accuracy +5, which I'm assuming that you want to use for the Accuracy.
Now, to compare, you close the gap in terms of Accuracy a little bit between Great Axe and Great Sword with the Accuracy on the sword itself, but if you really look at it then you'll note that you're losing 3 Accuracy just by equipping a Great Sword over a Great Axe, so really the Braveheart is only 2 Accuracy more accurate than the Great Axe equivalent, and still loses 3 Attack. Beyond that raw Accuracy/Attack difference, if you look at the weapons themselves you'll see that, just in terms of proportions, the Braveheart has a little bit lower delay (it's only 92.5% of the delay of the Neckchopper), but in terms of base damage you lose out considerably, with the Neckchopper adding almost 1/3 more damage just in terms of base damage (30% increase).
Last category between the two is Weaponskills. Great Sword weaponskills are, to be pretty honest, horrible at low levels. As a weapon it doesn't really come into its own until a player level in the 60s or so (Spinning Slash, which, it is worth noting, Warrior does not have access to). The weaponskills you have access to are all single hit or magical WSs, none of which are really going to do all that much more damage than a regular meleee hit. By comparison, a Warrior learns two excellent weaponskills very early on, Shield Break and Sturmwind. Sturmwind is your best raw damage WS until you get Raging Rush at level 60 or so. Shield Break is an incredible support Weaponskill, not being surpassed in that regard until Full Break, at 65. Sounds the same as Great Sword, I know, but really the difference is that you use "Great" weaponskills until you get "Incredible" weaponskills with Great Axe, and with Great Sword you use "Meh" weaponskills until you get "Incredible" weaponskills. Both of them wind up ok, but the first 60+ levels of using a Great Sword are rough territory.
So, basically, if it turns out we all look like a bunch of big jerks who are trying to tell you how to play your game, keep in mind that really, there is a difference between the weapons. If it really is that you just want to use a Great Sword for the looks of it, you'll have a faster and more pleasant time getting WAR to 30 or 37 and just unlocking DRK, where it's much less likely that anyone will give you any guff for using a GS.
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