https://www.planespotters.net/airline/Delta-Air-Lines

Delta uses MadDogs for LIT-ATL, average age 28.2 years or a little more than double Southwest's average fleet age. In the grand scheme, all meaningless figures. Those ol' things are on the list to be sent to the sandbox by Delta. No telling what they'll ultimately replace the MD88 with on that specific route; perhaps an old exNWA short-bus or whatever <shrug>. The corporate brain trust that makes all of those decisions also changes their collective minds about as often. We don't see the MD8x falling out of the sky (well, other than a couple of high-profile operator-error events from way in the past).

As wisely observed by LegalTender, this is not an apples-to-apples business. The fact that LIT is a thin outstation for both airlines just means your options are fewer (as you've noticed). As for which carrier is "safer"....YMMV.