The second set of tires does seem the most cost-effective solution right now - the second wheelset with the 10-speed cassette would be much more. You also might want to consider Conti Travel Contact, REI has them on sale right now. I have them on my Gunnar, smooth in the middle, but knobbies on the outside for gripping in corners or if you get forced off the road. These are great tires, and this is the best price I've ever seen for them. They would be fine for limestone as well.