Any wind flag is better then no wind flag, that said I could now state my preferred poison. Wicks with a dual vane are my favorite to shoot across, but they are a pain to take to and from the range because they don't lay flat and can fill up the trunk real fast. I was able to shoot across friends Flying fish single vane wind flags and loved them. Using a sail tail they worked very well.
Shop around you can find a set of used flags for less then ½ the price on new.
Randy