I've not done the two beaches as excursions. I travel to GC for a couple weeks every year and can tell you a little about them though.
7 Mile Beach is pretty close to Georgetown ( where the cruise terminal is ) there are several beachfront hotels and the beach is lovely. The further you go west, the wider the beach ( IMO ). I'm not sure what the excursion provides you with, but it would be easy to walk into a resort and buy drinks, snacks, etc. ( But watch out, EXPENSIVE is key- DH bought a single scotch on 7 Mile last summer and paid $16.00 or it!!! ) That was at the Marriott.
Rum Point is a lovely spot more towards the east end of the Island. It's a pretty long drive from the port. You'd head east and basically go around to the back end of GC. Rum Point has beautiful water, the water is shallow for a long way out which would be good for little kids. There are shops, a bar and a grill to get food. We didn't eat there, but ordered drinks on our visit and hung out on Hammocks for a bit.
The big difference would be that if you stayed around 7 Mile, you would have time to do "other things", shop etc on the Island. I'd guess a trip to Rum Point would take up most of your day.