Realizing and admitting your own inexperience is a remarkably courageous insight and may mean that you're just the kind of person who could actually become good at this. So much of building ability with horses requires being self-aware and having the flexibility to adjust what you're doing. Between your choices, I think that both could be good or bad depending on what you do with them. I can tell you that it is possible to train your own horse being self-taught and eventually get good results because that's what I've done. There are a few things I could say about it, and I hope that these help you in your decisions. :)
1. It is true what they say, green on green make black and blue. There's a saying I've heard attributed to the French, to the effect of "it takes seven falls to make a rider". You will fall. I've had a bunch of wrecks myself. The question becomes, how good are you at getting back up?
2. This horse isn't likely to ever reach as high a level as she may have if you'd had her as a baby and were already a master horseman doing (most) everything right. This is because you're going to make all your mistakes with her and teach her things that you'll later have to fix. She probably won't become a champion - though stranger things have happened. More likely she'll eventually become like those old lesson horses who have learned to just overlook their riders' mistakes and aren't super-refined but hopefully, are forgiving and will get along with you anyway.
3. Throw away the clock and the calendar. If you can do that from the beginning you'll be better off. You'll never be finished with her and the training of a horse lasts the entire lifetime of a horse.
4. Develop a love for the process of learning about horses and study as many different teachers as you can. Avoid the trap of believing that any one teacher or method has the answer if it goes against your intuition about YOUR horse. Learning from good teachers is extremely valuable but remember that you're the one paying the bills and your judgment is what's going to effect the animal long after the clinic is over and the DVD has finished playing.
5. When you find teachers that you like, make an effort to go and see them in person. There are a lot of things that happen in working with horses that aren't always shown on the finished product of a DVD.
6. Make an effort to get yourself on video. You won't believe the things that video shows.
7. When (not if) you feel frustrated or at a loss there are two things that have really helped me. One, realize that even when you become a great master you will encounter situations that will stump you again and again. That never ends and is central to why horsemanship is a lifetime pursuit. And finally, when you reach an impasse just realize where the responsibility lies. It is with you, not with your horse. Make yourself better and you'll improve every horse you come into contact with. Be the rising tide that raises all the ships. Or as Chris Cox put it, horse trainers train horses but a horseman trains himself.
Or alternately, you could just get the more experienced horse and maybe enjoy riding more from the start. That could be the wiser choice and if I had everything to do over again I'd probably go that way. But I do have a soft spot for those brave folks who become determined to do this on their own.