LotusEater
I brake for blue butterflies
I always end up with at least 20 in the bank before I get to Solstheim so Miraak is never an issue for me.>> If you can buy only one right now and the other later, I would pick Dragonborn. Reasons ? You will get more shouts, armors, weapons, enchanting effects ( Chaos = the best in the game ), an additional house, money and of course you will have the pleasure to kill Miraak and get his armor and weapon.
Also, you will get the Waking Dreams Black book which will allow you to change your skill tree distribution perks at the cost of Dragon souls gained by you after killing dragons. However, if you buy this one, make sure that you have at least 10 dragon souls available in your inventory to use them for those shouts that you will get, especially, the last Dragon Aspect shout that you must get before you ride the dragon that takes you to Miraak. If you do not have a dragon soul available in that moment, you miss the chance forever.
While Miraak is still alive, he will not let you suck a dragon soul so it is crucial that you start Dragonborn with at least 10 dragon souls available in your inventory. After you clean all the Stones, which it is a quest right before facing Miraak, you will be able to restore your ability to suck a dragon soul if you kill a dragon but do not rely on that at all because dragons may not show up and if so, it would be just one at the most. If you go back to Skyrim to suck more dragon souls, it would not work as well. So make sure, that you have those 10 dragon souls before starting Dragonborn.
After you kill Miraak, you will suck out of his body, 3 more dragon souls so you can use them right away at that place to reset any skill tree that you want.
In my opinion, Dragonborn is much better than Dawnguard but to face those vamps, you would have to need to be more powerful and the dragon aspect ( that you get in Dragonborn ) will make a difference when facing those vamps. Just my 2 cents ....
I wouldn't suggest banking ten souls... that's too many. Having three in your pocket is fine since you only need to unlock Bend Will to progress the questline. Also, Miraak will not give you three souls upon dying, he gives you 10 souls at least. Also, Miraak doesn't steal every soul, just most of them (roughly 2/3 by my estimation). It's still possible to collect souls after Miraak is unlocked. Dragon Aspect and Cyclone are fine Shouts, but not necessary to complete the quest.
Wow... Miraak has really gotten into your guys' heads.