.999 re-sells better than 90%, just like 90% is more desired than 40% and foreign silver.
Plus, bullion tends to weigh exact. Whereas, coinage has circulated for years and worn. Also, MS70 Eagles have sold for major premiums, and I think more than fair share of those from West Point.
I started collecting silver again because of investment/doomsday/economy/inflation survival and I prefer old coinage for that reason. American Eagles are beautiful though, especially gold ones, but if people started bartering with precious metals, it would be like having a hundred dollar bill with a doubtful outcome on making change.