I know others would be bale to answer this better then I, but I'll tell you what I can and someone else can fill in the blanks.
Both will let you drive on the street fine. with duels you will have more options as far as gearing goes (1 case in Hi 1 in low, both in low etc). with just the 4.7 you will have the same high range, but your low will be crazy low.
You have a few options, get the adapter and run duels with stock gears, or if you will get some more $ not far down the road, get the 4.7 now. Gear the extra case and swap it with the 1 in your truck to reduce down time of the truck. Then when you have the extra $ get the adapter, cross member, and do the drive shaft work to go duels.
