An upright vacuum needs to have a variety of attachments, be powerful enough to suck up the deepest of dirt, and be versatile enough to be used on carpets OR linoleum/hardwood floors.  I don't think you should look for a vacuum with a hepa filter, because it truly makes no difference.  I suffer from bad eye allergies and hayfever; the hepa filter never helped.  So really, you are paying more money for nothing if you are looking for a vacuum with one.  It makes more sense to change your air filter more often and keep vents dust-free, rather than worrying about the vacuum's ability to filter out dust and dander. 

I've had many vacuums in the past that started out working well, and eventually lost their umph. Sometimes it's been a year down the road, other times two or three.  What I've discovered is that bagless vacuums are both more economical and clean better than ever.  I just love my bagless Dyson, which meets all my needs. For the first time, it sucks away the dirt with such vigor that my carpet actually looks one shade lighter than it normally does! Makes me wonder what the other vacuums really did.  I love how the Dyson is lightweight, narrower than most vacuums I've ever owned, and its attachments are stored within the vacuum itself.  Good-bye to all the hassle with bags and having messy accidents when trying to disengage one to throw it in the trash.  Instead, the Dyson has a trigger that releases the cylinder holding the carpet debris, and I can just dump it over the trash can without any worries.  The best thing is, I don't have to replace any parts!  This baby has saved me tons of money, making my carpets look fresh and inviting.