I got the Retrax Pro MX because it is highly recommended, looks good and is very sleek only downsides is that the side rails have to be close to perfect, and the unit seems to move after it settles, nothing major but enough for some slight gaps for water to get through by the tail gate. You don't necessarily need to adjust it, but if it bugs you like it does me, this will be my 3rd time adjusting it.
I was going to go with the Bakflip but I like the function of this better.