I'm fairly new here too, so consider that when reading my reply.
My first 2 cooks were with water. Everything went very good, except for some bonehead errors I made. Both the butts and the WSM were very forgiving, so the cooks turned out well.
I tried sand for my next two cooks after reading much about it here. My observation was that the temps were easier to control with water. Maybe it was just me. As someone else pointed out, you have to ramp the temps up carefully with the sand because it is much harder to bring the temps down.
Water is never going to get above 212* so you have an excellent heatsink. Sand will get much hotter.
I'm not giving up on sand, because I think it could be very good, given all its other advantages. I just think a beginner will have an easier time with water for a low and slow cook.
Please note: I am not trying to get a sand vs water debate going. There are strictly my observations and I am a rookie. YMMV