The key point is: it is up to the dog to decide if something is an actual reward or not.
So you have to know your dog and see if he's working "enthusiastically" or not. If he's distracted, sniffing around, taking the treat without really paying attention to you and going back to its business it means that the value of the treat is not high enough.
Make it worth working with/for you.
I find it easier to work with home-made dog treats: pieces of chicken, or other kind of meat, pieces of hot dogs, etc. Even if dogs are not obligatory carnivores, proteins are always a good choice, chicken doesn't have too much fat, and you have total control on the quality. Then you can reduce the amount of dry/wet kibble you feed him.
To be more specific regarding your question: that way it is super easy to have some variety, and, from personal experience, I think it always helps because he gets extra motivation discovering what's new today. You
I'm buying different sorts of commercial treats just for my own convenience: I don't have anything to prepare, I buy only dry treats, that are easy to carry in my pockets, etc. Looking at the ingredients entering the preparation of these treats I've never been able to conclude that some are really healthier than others, so I just buy randomly, see if he likes it and in any case I really limit my use of these treats: doesn't make sense to feed him excellent quality food and then give him too much commercial treats of "random" composition.
A side note concerning other types of rewards: yes, praise and toys are good rewards but I use that only for informal training sessions (when I often doesn't have treats with me) or to reward a big chain of behaviours (for example: if your dog is heeling correctly, you can do some complex exercice and reward him with a toy at the end). But treats are really the reward of choice because it is convenient, it doesn't distract the dog too much, etc.
Adding a quote for Karen Overall's Manual of clinical behavioral medicine for dogs and cats:
Foods that are high in protein may help induce changes in brain chemistry that help the dog to relax, so choose a protein treat over a carbohydrate one.