I think you'd be fine using just the glass or porcelain. Can't imagine it'd make any difference in a long cook...Cooking vessel material generally only affects flavor if there's a chance of metal interaction (e.g. lemon curd in a reactive pot) or needs to be considered for heat retention/transfer (like a pizza stone) but for a long roast, you should be able to go with one or the other and be perfectly OK.

Con,
Thank you so much!
MJ