You're not losing anything by going to the 4.7s.  If you had the dual 2.28s, you get 1:1, 2.28:1, and 5.2:1.  With the 4.7s you get 1:1, 2.28:1, 4.7:1, and 10.7:1.  So for the money, the 2.28s get you 3 ranges, and the 4.7s get you 4 unique gear ranges.  I'd say if you have ambitions of slowly crawling over large boulders, save up the extra cash and go for the 4.7 gears.  If you're just looking for the extra grunt to get your rig down the trail, skip the 2.28s and just get a single 4.7 case and save yourself a lot of money.  You can always add the doubler later on, when you're ready for the x-member, driveline, and sheet metal mods.