One thing I like about the zip locks is once they really start growing you can cuff down the top and the "zip" part holds it there nicely. But mainly its the space saving, I can fit at least twice as many in a standard tray as I can 4" pots (plus a 5" tuber still fits nicely)
Be careful about letting the sprouts grow up against the plastic - that can rot them - just stuff some soil between the sprout and the side of the bag if that is happening.
And when they go outside to harden off either snip drain holes or cover them when it rains - I drowned a few last year, all of the baby roots died and they never really recovered.
