It really depends on what you can handle.
Newfoundlands are big, but not as big as a mastiff. They drool a lot, so you need to be prepared for that. They have a thick coat the needs to be brushed several times a week and will shed a ton. They love water, so it can be a challenge to keep them away from it, if you do take them swimming you will need to brush them before and after and make sure they are well dried. They are a more gentle breed, but still require consistent training and rules, they are still a big dogs and could hurt someone if they were to jump up or pull on the leash. They aren't that active compated to other breeds, but still need regular excercsie. Like most large and giant breeds, they do have joint problems as well as the risk of bloat so do your research on breeders and have a good vet fund available.
Mastiffs are also large. Some will drool, but it should not be as much as a newfie. They have a short coat, so still need some grooming but not much. They can be intimidating to look at, so make sure it is well trained and socialized and don't do things that can make it look worse (like a spike collar). Again, not that active but still need excercise to stay healthy. Joint and other issues as also there.
The main difference would be the coat care needed, the size and what they have been bred for (newfies for pulling in fishing nets, carts and such; mastiffs to protect the property).
Personally, I want a Newfoundland. You need to decide what is right for you though.
