Frankly, I've used the pale buckwheat malt in several beers, and find it to have very little flavor at all. It is easily overwhelmed by things like candi syrup, millet, wild rice, molasses, and even honey. This surprised the heck out of me, because the grain when cooked normally has a very strong flavor. In all the GF beers I've had that use buckwheat (Harvester, Green's, Glutenburg) I can NEVER pick out any buckwheat flavor. It also gelatinizes at a higher temp than millet, so is not a good candidate for a mash done without enzymes added. I do not recommend it.
In my experience, millet is the best base grain, bar none. If you're crushing it using a roller mill, I highly recommend double-crushing. My Monster Mill even on the tightest setting doesn't give a totally sufficient crush on first pass (which I learned the hard way, getting a very poor efficiency on my first batch done with the mill due to insufficient crush).