Dawnguard

One should be very careful when playing with Dawnguard installed on account of the random vampire attacks in cities and towns which can result in the death of someone you'd been thinking of marrying.  Anyone who is outside at the time of an attack is likely to rush in to fight the vampires, and also very likely to be killed if they do so because they don't have the skills or health needed for combat.  If you are ever near a town or in a city at night, or late evening, be careful and keep your ears out for sounds of combat and get over there as quickly as you can.  It's also a good idea to save before exiting any buildings at night, and to fast travel to the outsides of cities to make sure you enter them during the day time when an attack is much less likely.  Unfortunately, though this has mostly worked in my experience, I have in fact left a city with everything looking fine, been away a few days and then fast traveled to the stables, waited there till morning and then entered - and found the remains of a deathhound and an ash pile by the gate.  It doesn't make any sense that this could have happpened while I was gone since essentially that section wasn't in existance since it wasn't loaded, but.... I can't deny the truth of what I just saw.  I didn't see any bodies of townsfolk, luckilly, but still.... it is unsettling.  So if you want to make sure your potential spouces stay alive, be very very careful and save often.

Where to Live

Some of the Marriage Partners own property and, once you're married, their house becomes a safe place to store things, which can be particularly handy early in the game before you've bought a house of your own.  But don't make a rash decision just because you need a place to drop stuff off.

Even if you're not planning on using magic, I'd highly advise taking a carriage to Winterhold and joining the college as soon as possible because you will be given a free room with a bed and several containers which are safe to store things in, and will also have acess to an enchanting table and an alchemy lab with a lot of free ingredients. Don't be scared off by Faralda's test before being allowed to enter the college.  If she asks you to cast a spell you don't know she will give it to you for 30 septims which is much cheaper than you would be able to buy the spell book elsewhere, and she is likely to choose 'Healing Hands' which is very useful if you have a follower (if you're looking to get a particular spell cheaply, save before talking to her and just reload till she asks for the right one).  Joining the college also allows you to buy and sell with the various professors, as well as one fellow student, Enthir, who is guaranteed to carry the hard to find Deadra Hearts and Black Soul Gems.  Although the Professors may not sell much you're interested in, if you get the Merchant Perk at level 50 Speech you'll be able to sell anything to them which makes the college a very convenient markteting stop as there are 6 people to sell to all in the same place and you can wake them up at any time of the night to trade with them (they are also a good source of soul gems for recharging your enchanted weapons).  

But now, back to homes.  Even if your intended spouce owns a home, they may only have a single bed in it which makes sleeping there difficult if they are sleeping at the same time you wish to, however you can sleep in their bed whenever they are not in it and still receive the 'lovers comfort' which makes skills increase 15% faster for the rest of the day.   You also have to be careful about containers and property in their home that may belong to someone else who also lives there.  In most cases it may be best to move them to your house once you have one - they will always have a moving speech option available so don't worry about your first choice of homes being permanent after the wedding.  Once they move into your home you will be able to buy and sell things with them, usually like any other general merchant unless they were already a specialized merchant before you married them, in which case they'll have the same stuff they always did.  Also, if you want to adopt children, you will most likely have to buy your own house because you have to have an extra bed and chest available for each child.

Something which may make it more attractive to stay in your spouce's home is that when living there they will continue their normal daily routine, so you can find them in the pub or wandering the market or tilling the fields or whatever else they do.  Once they move to your home they will just be there all day which doesn't really matter, unless you're like me and feel like maybe they miss their old friends :)  It should also be noted that in the houses you can build yourself, your spouce is apparently supposed to go sit in the roof of the Armory during the day, but if you don't build the Armory they will just hang out on the side of the house where the stairs to the roof-top patio would have been - which makes them particularly vulnerable to attacks from bandits or wolves which happen from time to time at the Hearthfire Homes.  I should also note that sometimes, though rarely, the bandits actually can enter your home: for instance if they attack while you are on the roof of a tower and you need to enter the house to get down to your front lawn to fight them, you may end up running into one coming through the house to reach you up on the tower.  There is also a quest in which your spouce can be kidnapped by bandits and you will have to go and rescue them.  Thus, living inside a city is a much safer option.