The 70/300 is a bit soft on the long end but sharper than anything else in the range.
both would probably perform about the same as you really need to stop down to f/8-11 for maximum sharp results.
If you like the 18-200 - personally I would suggest going with 28/300 - same range you are used to.
That said, if you are pixel peeping for sharpness, your best bet is to go with a couple 2.8 lenses with less zoom or with primes. Doing a quick analysis of what focal range you shoot most would give you the best idea where your money should go towards - long, middle, or short if you go that route.