I think it depends on what you're going to run the deck on. Are you talking a ZTR or a Tractor because it can make a big difference. I'm assuming the one acre were talking about belongs to you and you're not using the mower for a business application if so then Deck size is really irrelevant.
Your time gain with the 48" over the 42" will be minimal at best. I can tell you I run both a 44" and a 48" ZTR, although the 48" is somewhat faster the 44" is cheaper to run, fuel cost is something you may want to consider in your choice.