Been using the Zweihander since day 2 and now I have the option to upgrade it between either the Zweihander Raw or Zweihander +6. They both cost the same but have slight differences. The Raw does like 15 more dmg but I'm not sure if you can upgrade it after you make it raw. While i'm sure that you can upgrade the +6 more. What should I do?
Actually, Raw weapons have worse base damage and scaling than Normal+15; however, you can max out a Raw weapon using only small and large shards of titanite (which are both infinitely purchasable). Basically, it's what is in your supply. You'll probably only get enough titanite to get one or two Normal+15 per playthru, but you can get infinite Raw weapons. It's a great way to get multiple fairly strong weapons before determining which you want to make a +15 or a Lightning+5.
Also, you can create these very early into the game, putting you ahead in power. So, in the long run they are bad. But in the short game they're fantastic.
Edit: You can also drop it back down to +5 and move the same weapon towards Normal+15 later. All you lose is a couple of large shards which can be purchased from a vendor in Sen's Fortress.
Demon's Souls/Dark Souls Stat and Weapon Calcs http://sites.google.com/site/darksoulstats/