This is extremely difficult to answer for outsiders who are not familiar with your personal shooting style.

Smaller zooms used to have better MTF charts (and prime lenses even more so), but that has changed - in fact, Hasselblad's zoom lenses have equal or better performance than some of the primes they cover. Of course compromises must be made, but I wouldn't worry about them too much and be more concerned about the practical side of things.

Sometimes, two lenses are better than one - perhaps it is the additional reach of 300mm, just bringing the one you need, or having faster speed on the short end. But not always: if you often find yourself wanting to go from long to wide to long rather quickly without the luxury of multiple bodies, a single lens that covers the full range has huge benefits to swapping lenses. Or when you're on day-long trips and that second lens adds weight and bulk to your pack. Or you're shooting in environments where changing lenses is difficult (thanks to salt water spray, snow, sand, or dust). And so on. So the decision isn't always about having the best possible optical performance - it is practically always more complicated than that.

That said, I would go for the 18-200. If you're not sure what you're after, this will allow you the flexibility of exploring photographic possibilities without losing time constantly swapping lenses.