That all depends on the time of year and the forage they are feeding on. As fish are no different then humans, they eat many things under different conditions and circumstances. What they eat depends on changes of the seasons and availability of forage.
The general answer would be to 'match the hatch'!! What that means is: whatever baitfish are in the area, find a lure that will match or duplicate it in size, action and color, or better yet, catch some of those baitfish! Sometimes this works fine. Keyword is sometimes!!!!! Can you imagine eating hamburger 6 days a week. I bet on the seventh day your sick of hamburgers and would not eat one on that seventh day. You might eat pasta on the seventh day, because you're so sick of hamburgers.
I was feeding snook scaled sardines for 3 days, and it was just fine for them for those 3 days. On the 4th day those snook would not touch a scaled sardine. I presented a pinfish to them and they ate it. That being said you first should have an assortment of lures with you for both species. The lures should cover the entire water column. Example: topwater, suspended bait and sinking bait. You should have them in different sizes, colors and actions. A jig and spoon is always a must in a tackle box because you can do so much with it. Snook and reds both will eat them.
A redfish is a inferior eater, meaning the lower part of his mouth is shorter then the upper. The term "Tailing Redfish" comes from the redfish routing down in the grass smelling with his nose and eating what he finds. His tail sticks up out of the water. They mainly feed in the lower water column. Therefore, one of the best baits for a redfish would be a Z-Man Procure bait or a Berkley Gulp because they smell and are soft and easy for them to pickup with their inferior mouth. There is a reason they are the #1 selling baits for redfish in every tackle shop in the State of Florida. Running in second place would be a gold spoon for redfish.
Now for Snook. They are a superior feeder. Meaning their lower mouth is longer then their upper mouth. A snook - when attacking a bait - will suck in the bait, water and air in one fast pop. They mainly will feed in the middle to upper water column and a baitfish imitation is excellent. One of the best baits for snook is a XL Mirrolure Mirrodine in green, white and silver for this time of year (Spring/Summer). Second would be a Zara Spook Jr in Bone or Bone, silver and white.
If I had to use one lure for both species. I would use a Spro Jig (bucktail) in 1/4 to 3/8th ounce, color: white or white and green. Now winter is a whole other story with baits. Always remember there are no rules in fishing and these are suggestions that have worked for many anglers!
Always leave a place better then you found it!!!