"a Water Fast or a juice fast?"I think this is really subjective
As far as meeting your "main goal...to lose weight and to detoxify" both water and juice fasts will adequatley help meet these goals.As they are 2 different methods they will however differ slightly in how they achieve theses results.As you already know Water Fasting tends to be harsher and quicker in eliminating toxins with greater and quicker weightloss whilst juice fasting tends to be a milder cleansing/detox method (if not resulting in more energy)and not as drastic quick weightloss results.
Poor thing!you sound really overhelmed about this.How about alternating juice and Water Fast days this way you get the best of worlds energy and quick weightloss.As for the length having fasted before,I'm sure you have an idea of your body's abilities and limitations keep that in mind as you figure out what you'll be aiming for in this upcoming fast.Also just remember to take it each day at a time.Also remember that even when juice fasting your are detoxing so you will have some bad days as well.Try to calm down and just figure out what'll work best for your own situation and go for it; you'll be amazed at what you can achieve.I also have a post titled Day 20 of a Water Fast feel free to take a look and see what you could expect and some tips.