outbound, make sure you have enough time for you+bags to both make it on the ATL-MCO leg
i never had time outbound to try a DL lounge at ATL, but the transfer is easy and there's far less risk of the long queues MCO can inflict on international arrivals
inbound, at MCO there's a new lounge to try if you've got Priority Pass, otherwise i'd consider heading for the Outback rather than the DL lounge
at ATL, once in the international terminal, the DL lounge is a bit better than their average effort, but still nothing special and i find it even more unpleasant than MCO once it gets busy, though the ability to go outside and breath in the jet fumes is a plus if the weather is ok, again i prefer to head to a restaurant instead, the italian one upstairs is pleasant and roomy
for duty free, i used this on my recent IAD trip, worked very well...
https://www.dutyfreeexpress.com ...just remember to pickup as you reach the jetway!
on the 333, avoid the D/G seats as they are shorter and have other disadvantages vs. the A/K seats
the other plus with going via ATL is extra tier points!