Today, Jan 8th, I can't find any US dealers selling 90% at spot, although my favorite, SD Bullion is darned close. Stackers shy away from junk because the math is a little more complicated. Units are face value instead of weight.
The biggest problem with junk is that it is worn, and therefore below minted melt value. Most reputable dealers will account for this. For instance, the melt value of US $1.00 is .7234 ounces of .999 silver. Most dealers will adjust this to .718 or so. For reference, the last batch I bought from SD Bullion in September, the multiplier was .715. Weighing the total batch confirmed this.
I prefer junk as a primary because I can purchase it with no premium, and often end up with collectable coins.
I have nothing against bullion coins, but prefer government issued like Eagles, Maples, Libertads, Britannias, etc. Some now have numistatic value in addition: especially the Libertads with low mintages.