Hi, I use hard gels to marble with, the softer soak off's have a thinner viscosity. Your looking for a nice thick gel, so the colours won't bleed and split to much.
if you don't have joy with the gel your using try popping it into the fridge for 20 mins. this will help you work with it. I only recommend the fridge method for the 'arty bits which you sandwich between the base and capping layer.
All art work with gel is best when the viscosity is thicker, but I use a range of different ones like Geeg said. I tend to look at colours I like; I love IBD; for me this is my first choice for gel, but it doesn't have a really nice dark green, so I use the green from Creative, their red isn't half bad either. I am in complete awe of Astonishing cover pinks and some of their glitter gels, and the French it from Ez Flow is the best for flowers. I have actually custom blended my own yellow using a yellow from IBD's tropical, white and yellow pigments.
What I am trying to say is that you need to build your own collection, and find your own favourites. and the only way to do this is play, play, play.
hope this helps.