An easy way to find out which does more damage is to equip both, go inside to fight them. Smack one and note the damage, then switch and smack him again with the other one. Use whatever is higher damage
Generally the thing about O&S is you want to be faster, not heavier. The reason being that locked combat makes it more difficult on the first part because theres two of them that can easily flank you. Thats why hit and run tactics work so well against them. Avoiding the higher damage attacks is also easier.
That said, Ive beaten them with fast, medium,and fat rolls solo and I do highly recomend you beat all the bosses solo on your first run, it makes it feel sooooo much better when you finally win. So its not impossible to do, Im just saying whats easier in my experience
If you're really sold on staying heavy then keep them both in view and watch for attacks. After one, move in, two hand it and smack them once, then get your shield out and back off. Do that till Smough is dead
As a side note, if you want a more sturdy parrying shield keep killing silver knights for a silver knight shield. That thing is amazing when fully upgraded