in my opinion, 必须=must, 非得=have to.

Examples: John非得要牛奶才要喝。John cannot accept any kind of drinks but milk. John必須喝牛奶。John must have milk. There's still slight difference of these two phrases.

非得 literally means "cannot be done in any ways but only in the way of", emphasising on ruling out possible options, and 必須 suggests the only option to accept.

must and have to

必须比较正式。非得多用于口语。意思一样