Are certain NPCs better to marry compared to others? Or do they all provide the same benefits regardless of who they are/where they're from/what they did (as long as they love me)?
Yes! Some NPCs make better spouses, but only as far as utility.
- Spouses open up a "store" and provide you with half of the earnings (depending on the character it could be 100 to 200 gold a day). They will open up a store regardless of where you choose to live. Spouses that previously were shopkeeper NPCs have the stock of their previous shop. Spouses that didn't previously have their own shop open a "pawnbroker" type shop, which can have some nice things in it, like Ebony Ingots and enchanted apparel. Depending on who you marry, you might want/not want their previous shopkeeper stock.
- Spouses that previously provided training continue to provide training after marriage (ex. Farkas). Money you pay them for training temporarily becomes part of the money pool for their buy/sell window too, if they open a shop for you.
- Spouses that could previously be followers/companions will continue to have that option. Not all spouses are able to be followers (ex. Willhelm is not a follower). If you plan to have your spouse follow you, you might want to marry someone who is the class you want. (ex. Farkas is a warrior/tank, Onmund is a mage).
- Spouses that previously provided quests continue to provide those quests. It might be convenient to have that questgiver live in your house instead of their previous location.
One NPC, Angrenor Once-Honored, dies naturally as part of the game unless you marry him.
Other than above, race, appearance, or gender don't make a better/worse spouse. All spouses will live with you, cook, and provide the well-loved resting bonus.
Unfortunately, the marriage element in Skyrim is incredibly shallow. Who you marry is up to you. The only difference in who you choose is that some available bachelor(ette)s can be taken with you as a follower, and will fight with you. Regardless of who you choose to marry, your spouse will always open up a shop, giving you 100 gold per day. Ysolda in Whiterun, and Aela the Huntress of the companions are two good examples. Ysolda will open up a shop, but that's all you can do with her. Aela will open up a shop, and you can have her follow you into combat. Those are the only two differences in who you choose. Chances are, if you could already have them assist you in combat, the option will still be available post-marriage. If you couldn't get them to follow you into combat, then they just sit around the house and give you money.