Both options will give you far more silage than you will need for that size bag.  I'd estimate the 40 acres at 600 ton and the 15 acres at 400 ton.  Corn yield affects tonnage far more than quality, so that won't factor in.  In 2012 when we had 1/3 to 1/2 of normal yields, the silage tonnage was correspondingly lower but the energy level in the silage was still 80-90% of normal.
I like Crete's idea of taking the earlier corn and following with rye - it wouldn't let you graze stalks, but there is not much to graze after chopping anyhow.