found a good shop if they are willing to do an exchange like that
A 700cc sized wheel will go faster compared to that of a a 26 inch, but as you have found it comes with its draws back a 700cc rim can't stand up to aggressive riding and you feel more of bumps in the road.
I'd say you are making a wise choice switching of that a 26 inch rim. Sure it might not be as fast to the that of a 700C but you will be able to go off road and take short cuts
What it comes down to is it speed or durability that you are after . 700cc = speed and feeling bumps in the road, or, a 26 inch rim = durability with 10% speed loss and less road feel.