Staying in Murrell's Inlet, you're just down the street from Express. Coastal is in North MB so it could be an hour drive - especially in the afternoon/evening, a lot of the tourist attractions/dinner theaters are in that area - and there's really not enough alternate routes when the main freeway has a frequent delay(twice in the 5 days we drove in that area).
Notice that the divesites listing for either op are almost identical. Except that Coastal does more deep/all day trips. Personally I'd save the drive time and dive with Express. Especially if you're diving with the kids, they do a lot more shallow stuff. The DM stays on the boat unless you need a buddy though. Good group of DM's/staff also.
Know that most diving in that area consists of the dive site surrounded by flat sand bottom. And it can be pretty murky at that time of year, it was when we were there in late June - probably 30-40' max vis. The water will be bathtub warm by then also. The only reason to wear a t-shirt/skin is there can be small jellies in the water. We dove the Perry and some other wreck, plus two other sites, all were average.
If the kids are too small to dive but want to see the fish, Ripley's Aquarium at Broadway on the Beach in North MB is better than I expected. Lots of sharks and one of those huge 2-3 story tanks. And a lot of restaurants/stores and other entertainment at B on the B, even fireworks at night as I recall - you could easily spend 1/2 day there. Traffic was really bad up there at night though.
If you're a golfer, it's golf nirvana. But it's humid as h**l that time of year. There's some great restaurants in Murrells Inlet also, people drive down from MB for it.