Great combination, but very different!
Similarities (in general): Both like water, both like snow, both like to retrieve in water, both have long coats, both are smart (in different ways however), both like people (including children, generally), both are velcro type dogs, both like to cuddle. Both have hip/elbow problems. Both tend to have allergies. That's about it.
Differences - Goldens (in general): Goldens stay much more energetic for years...up to 8-10 years. Goldens are super chewers and are known to be destructive when young. Goldens move faster, all the time. Goldens react to commands faster when being trained and once trained, Goldens want to please their people ALL the time. Goldens are "street" smart...or clever and have quick reactions. Goldens have incredible "noses". Goldens are more agile. Goldens fight more viciously. Goldens usually die from cancer, at any age. Goldens, if from good lineage, have excellent temperaments, yet are not generally what one would describe as sweet natured and gentle. Goldens are always alert. Goldens get over "hurt feelings" in about 3 minutes. Goldens need to be trained to be gentle. Goldens demand a lot of "hard" exercise every day (at least 45 mins 3x a day - more is better). If they don't get it....you've got a huge problem.
Differences - Newfs (in general): Newfs become low-keyed between 2-3 years of age. Some are calm from birth. Newfs think about what they're going to do before they do it. They watch you and learn how to do what you do. Newfs are sneaky and you don't know what they've learned until they "do it" out of the blue, when you least expect it. It's usually something you didn't want them to know how to do. Newfs are patient. Newfs don't destroy everything they get their mouths on every minute of every day for the first year of life. Newfs DO destroy things when they're bored or mad at you or when they just get a whim. Newfs can hold a grudge for months or longer, especially if you hurt their feelings. Newfs are intelligent. Newfs innately know when to be gentle. Newfs adore children. Newfs don't always want to please you, but they do always want to be with you. Newfs are super strong even when very young. Newfs are strong willed when they know they're right. Newfs demand your attention when they want it, otherwise, they're happy just being there by you, and if they're sleeping they prefer you leave them alone. Newfs only need "hard" exercise a couple of times a day for about 15 - 30 mins. Newfs are more sensitive to their environments, both mentally and physically. They suffer many more illnesses due to these sensitivities.
Both are wonderful breeds. Each dog, of course, has it's own personality traits, it's own behavior pattern, and it's own energy level. All are different.
But all in all, in my experience, these are the basic differences between the breeds...in general.