Great personal experience!
Upgrade to Large Wheels (18.5" x 8.5")
Absolutely get the large wheel upgrade! It's cheaper when you order the dolly than to retro-fit. These wheels are amazing with a superior no maintenance bearing system. Launching my 130 lbs Megabyte on an non-improved sand launch was a piece of cake. I really wish I had these wheels with my much heavier Weta trimaran. Well worth the upgrade charge!
I got the upgrade for our dolly for a dinghy with an outboard. The wheels handle the heavier outboard load nicely and float over the fine gravel and sand at our beach well. They do add width to the dolly which makes maneuvering a little more difficult but worth it for the beach performance. Our other dolly has the regular wheels on a small sailboat and it works fine on the same beach, but is a bit harder to pull up the beach at low rise when it's steeper and a longer run.
I purchased the 12' dolly with the large tires for a Mercury inflatable....it works great !!! Easy to assemble, and very easy to launch and retrieve the boat. Large tires very handy for use in sand.