Most of the posts so far have it right: the Dual is more solidly made, less temperamental, etc... than the somewhat finicky Zero 100, which seems to have suffered from greatly variable quality as it came out of the factory. Get a good Zero 100, and maintain it, and you'll be okay, but it seems most are not good and require regular maintenance. Restore the Dual once, and you'll be set for years. That said, get a good Zero 100 and you'll also probably have somewhat better sound than you'd get from the Dual... but there is no guarantee of it when you're getting an "unknown" Zero 100. If the Garrard was at all abused and the arm bearings have loosened, then it won't sound so good, and there is no way to restore those bearings once they are damaged.
The Dual is the "safer" choice, for sure. I think I'd stick with it.
On the other hand, if you ever find a Garrard 301, 401, 4HF or Lab 80 TT to swap, don't hesitate; they are all better than your Dual. Likewise, if you find a Dual 1219 or 1229, those would be upgrades. But there is nothing wrong with your 1019; keep it and enjoy it, until something definitely better comes along.