Congratulations on your marathon...it will be a wonderful experience. I would say just experiment with some different options in training to find the right combination because everyone is different.
I attempt to avoid the overly complex or highly sweetened sports drinks because they tend to cause stomach upset or some sort of other disgestive issues. I also stay clear of those with added energy boosters popular right now because there are side-effects. Basically, you might try watering down the selections that you try because most are concentrated with 2.5 servings of the ingredients per 32 oz., for example.
Again, it depends on your tolerance level for certain things and many sports drinks are similiar. I have tried an endless variety of products. I would say that Vemma Thirst and FRS are okay, but they also fall into the complicated realm.
Sometimes, I have found that simple is best, so just mixing honey with water is a great option. Another tip is real carrot juice with a splash of apple juice (this would be pre-long run or recovery drink).
Wishing you all the best in your training and upcoming event.
Have you tried Nuun? It provides the electrolites you need, but it has a very light taste, so it doesn't feel like you are drinking a ton of sports drinks. That's the only thing I'll drink. My favorites are the Strawberry Lemonade and the Lemon Lime. Half a tablet per bottle of water is usually what I use. I found it at my local running store. It's a little on the pricy side, but well worth it. HyVee also carries it. I haven't seen it at Walmart.