I suspect the answer is get the one that matches your situation best.

 

I encounter both hard paved surfaces and sandy surfaces.

 

I know you can get the wheels separately too but not sure how quick and easy it is to change them.

 

Thanks for any experience with these.

Changing wheels is a easy as pulling a pin and swapping them out.  As long as your kayak isn't on the cart at the time you do it, it should be really easy and take less than a minute.  

 

But I wouldn't worry about changing wheels.  Either wheel will do fine on hard surfaces.  No real difference there.  The beach wheels will do better on loose sand.  But on compacted sand, they'll perform the same.  The airless tires won't need to be filled up, so there's one less thing to worry about there, and since they're slimmer they'll get through thick weeds and around large rocks easier.  But for 99% of the stuff you encounter, they'll likely perform the same.  Even if the beach wheels start to get low, so long as they aren't really flat, they'll still pull fine.  You're not traveling huge distances or at high speeds, so it's typically not a big deal.  And the wheels don't generate thrust or turning forces, so it doesn't really need grip.  What is a big deal is getting everything balanced and secure.  

I've used the flat wheels on wet sand and they have a tendency to dig in especially if you're fully rigged up. Bankc did a good job of covering the pros/cons, if you're using them around sand, definitely get the inflatable wheels.