Schwalbe Marathon Plus vs Continental Gator Hardshell
There are reviews of both tires on bicyclerollingresistance.com: https://www.bicyclerollingresistance.com/tour-reviews/schwalbe-marathon-plus-2015 and https://www.bicyclerollingresistance.com/road-bike-reviews/continental-gator-hardshell
The Continental Gator Hardshell is considered a road bike tire, albeit a very puncture resistant one. The Schwalbe Marathon Plus is a touring tire. It’s heavier, available in wider versions, more puncture resistant and has a bit of tread.
If you need/want more than 32mm width you are out of luck with the Hardshell.
Otherwise it pretty much boils down to personal preference. Both are high quality tires with their own strengths and weaknesses. Personally I can live with patching a tube every month or so. At the same time I like to go fast/far and light. The Gator Hardshell is pretty much the heaviest, slowest tire I would even consider using. The Schwalbe Marathon plus is way too heavy and slow for me.
I’m currently using Continental GP 4 Seasons on my travel cyclocross. In the city I have about 2 or 3 punctures per year (2*6km commute). I can live with that.
If you need/want all the puncture resistance you can get there is no way around the Marathon Plus.