Judging by comments by Macrumors users, and analysts polls, the biggest loser will be RIM and some Android users, from within Verizon. Sure there will be quite a view switchers back to Verizon from ATT, probably in the millions, but there is a lot of pent up interest from BB and non smartphone users that wanted a Verizon iPhone...from within Verizon.
I also think the attraction of moving to Verizon because of 'unlimited data' is mostly PR..that will work with some. But every poll I've seen where people are honest...they use less than 2 Gb a month. Verizon does have a hidden cap of at least 5Gb, even though they call it 'unlimited'.
It really is a mute point. If you wish to estimate how much data you may, I found this
data calculator very useful