I am not too sure given the Welsh Gov recently acquired extra Mark 4 carriages. My presumption was they would operate all of the trains on the route given they have enough for at least seven sets.
Yes, the 158 has a few more seats, and, yes, the toilet thing has been mentioned about a million times. However, the capacity is basically the same. My point still stands too. An hourly service between Aberystwyth and Shrewsbury and potentially onto Birmingham would be heavily favoured by route users over a slightly larger capacity train on a two per hour service.