Are there any brands of twin wall polycarbonate that should be a avoided? Or one that is considered "the best brand available? It's hard to judge by looking at internet pictures. Thx!

Not sure about specific brands, but I would advise that you purchase poly twin wall that is UV treated on the exterior side to delay discoloration for as long as possible, and a drip guard coating on the inside to prevent condensation from dripping on you and your plants.