I do a lot of production work, portraits and corporate stuff mainly (gone through about 5 rolls of 10" paper in the last three weeks) so the rolls made sense for me. The 4900 is definitely a monster (but very impressive when a client walks in!) and you need the room for it, especially since opening the roll cover means you can't place it directly against the wall. I also looked at the ink costs which were a little in the 4900's favour if I remember correctly.. but with the difference in price you can buy a lot of ink! When it comes down to it though... the paper is really what costs the most, so you want to be able to use your paper efficiently.
I was also waiting for a 17" printer that didn't require the swapping out of PK and MK inks. I love printing on matte papers, especially B&W, but for my production stuff it's mostly Premium Lustre. This way I can move from one job to the other quite easily. If this doesn't apply, or bother you, then the 3880 sounds like the way to go.