Weight difference is negligible at best.
Ride whatever floats your boat, but CT are warmer without floppy socks required. In really cold water those neo socks work better in a closed toe boot! Socks in sandles is a poor solution on land.... no dif on the water.
Sand is sand. It doesnt matter if your toe is open once there is sand or a shell in the arch or heel, you gotta stop and empty it. It doesnt just make its way up and out your toe while you ride. It can however make its way in if you happen to stop and stand in the shallows for even a couple seconds in OT boots. I find way less sand washing into my close toe boots than did in any of my OT pairs. Especially when puting em on in shallow water.
Other reasons for close toe. More structural integrity to the boot, No chafe on the top and side of your big toe. no risk of slamming the forefoot through the open toe. You might not have had that one yet but you can look forward to it.