First off I would like to point out that everyone of your questions has been answered repeatedly below and in previous pages of this forum.
With that said, I'll try to answer most of your questions here. There is no way for me to tell you which would be a better match for you as far as burm or conda. I can tell you that I feel green condas require roughly twice the experience and dedication that burms do. They are by far the most challenging huge boid that I have worked with. I have no idea what your experience level, age, living situation, etc. is, so I'll leave that decision to you.
A neonate green will do fine in a 55 gal. but will outgrow it soon. Not as fast as the burm but still pretty quick. The 6' cage you mentioned is only going to be suitable for a male green. Green anacondas have the largest sexual dimorphism (size difference between the sexes) among all the snakes. A male would be a good introduction to greens and if he works out you could always get a female later.
Neonate and juvenile greens will do fine when started on rats every 7 - 10 days and after approaching 8 - 10' mark will do fine on an every 2 week feeding of appropriately sized rabbits.
You do not need a large water tub for greens. If you would scroll down a little ways in this forum you will find a good thread about the need for water tubs. I also never mist my greens. They do fine with the humidity between 60 and 70%. She sheds in one piece everytime without misting.
A human heating pad is not going to be appropriate for the cage. Especially if you have a large water tub and are misting all the time. They are not meant for wet environments. My anacondas prefer temps almost right at 82F and only slightly warmer after feeding. I would be suprised if you ever seen the snake laying on the heating pad for any length of time but I could be wrong.
Finally the most important thing to consider when thinking of getting a conda is the place you are thinking of buying one from. There are numerous wild caugt neonates being offered that are generally not worth even the $75 they are being sold for. There are exceptions but not many. As a newcomer to greens I feel you will have nothing but bad luck if you start out with a wild caught conda. Spend the extra $ and get a good captive bred one. I know Kelly is sold out and cant think of a source for them right now but they're out there. Just don't be tempted by all those wild caughts in the classifieds. If you can't find a good captive bred conda then get a burm. They are a lot more forgiving then condas (except for cool temps. I think the condas are more forgiving in that regards) and you generally have no problem getting them to eat anything.
Anyways I'm at work and have to go
Good luck whatever you decide on and keep us posted,
Arik