dawnguard adds some items (most of which arent impressive), a questline, and a few new dungeons. and annoying vampire attacks.
dragonborn adds an entire new island full of dungeons (i think theres like 50). lots of sidequests, the mainquest is shorter than dawnguards, but so what? the side content alone dwarfs dawnguards total content easily. it adds alot of new items, alot of which are super cool and unique and powerful.
Any new spells in either dlc? especially useful ones? If so are they sold in stores or found?
Dawnguard added mainly Restoration spells that do sun damage. There are also heal undead spells, but I forget if those are also restoration or not. I believe all the spells in dawnguard can be bought in stores, but I'm not certain. I have not played Dragonborn, but I hear it added a few more spells too.
Dawnguard added a "summon horse" spell which is pretty amaze balls. And crafting weapons from Dragonbones is pretty neat also. Dawnguard doesn't get as much as credit compared to Dragonborn as it deserves. You can't go wrong with either.