^ either one of your suggestions will look good. i think you should go with what appeals to you more.
personally i like the darker colors. what substrate are you going to use for your water feature? that may help you decide.
is the water going to come all the way up to the platform? can it? are you going to come down with them a bit more so the turtles can easily access them? some cool pieces of wood jutting into the water would be cool and could possibly add just what they need to get out. is that the root structure you were talking about

? if so great idea! i think it would look good. :P