If only a choice between the two, go with the higher NRR rating.
If looking for other options, consider the Peltor TacticalPro
The big reason to go with these kind of muffs is to be able to hear normal conversation while still protecting yourself from the noise of gunshots. What I've found is that doubling up on muffs with ear plugs prevents me hearing conversations, which was the point of the muffs in the first place. So if you plan to double up all the time, I'd just go with a non electronic set of muffs with a 30+ NRR.
I own the Peltor Tactical Pro's. If someone's shooting a .44 Mag or higher at the indoor range it bothers me, but I can just shut off or tune down the microphones and it's reasonably comfortable - no need to leave the hot range to remove my muffs to stick in earplugs. I'm not sure a NRR 24 will be as effective under those circumstances, but YMMV.
One disclaimer: I'm doubling up with NRR 33 plugs, same as what I use on the motorcycle. In the next couple weeks I'm going to try doubling up with earplugs more designed for concerts - NRR 20, that reduces sound evenly across all frequencies - and see if I can find a happy medium where I can cut down the heavy caliber noise further but still hear people talk.
One other thought: Peltor sells a gel ear seal kit
to replace the existing foam or foam/gel seals on their muffs. This is reported by some to boost the NRR rating a bit because of a better seal to the head. Not sure they are worth it - the Tactical Pro's already have a decent foam/gel seal, and if you are going to put those on the 6S or 7S, you just boosted the price past the Tactical Pro's.