American Staffordshire Terriers and American Pit Bull Terriers are not the same breed, however there are a lot of similarities, same with mastiffs. I am actually in a similar situation. I have a female golden retriever puppy and my husband would like a mastiff for our next dog. I absolutely love APBTs. I grew up with them. They are wonderful dogs who have a bad reputation because of bad owners.
Any dog can be vicious if not raised and trained properly. I have met many very very vicious chihuahua's. The difference is a chihuahua will make your finger bleed - a mastiff will remove your hand.
Some breeds known to be powerful breeds are dobermans, rotties, pit bulls, and mastiffs. But, I have been around a LOT of dogs and the only ones who have ever bitten me have been untrained, dominant, aggressive small dogs. I've never been attacked by a "bully" breed.
I would caution you against a powerful breed like a mastiff because the temperament would be quite different from your golden and that can cause problems. With extensive training and consistency you could teach any dog to get along with your golden, but matching temperament and energy levels is very important.
You may want to look into flat coated retrievers, or if your partner wants a "stronger" breed, look into hovawarts, they may look similar to golden retreivers, but they are actually closer to german shepherds. They come in blonde and black/tan (rottie colors).
If you're trying to convince your partner to get another golden you may want to point out how wonderful the one you have now is and talk about getting another one from the same breeder.
Just make sure you think long and hard about your decision and take into account what you want, what your partner wants and most importantly, what is best for your family, including your golden.