If you want a quick upgrade and don't want to screw with not only having to mess with the driveshafts, the adapter, finding a case then go with the single 4.7. If you want the choice of stock gearing and a crawl option and don't mind reconfiguring the driveline go with dual stock geared which will put you just about at a single 4.7. If you even for a minute think you may want a dual with the 4.7 in the rear case then I'd do it now not later. Having the longer front driveshaft does have its advantages when going duals. Personally I'd go single 4.7.
I'd only go with a single 4.70 dual case if you have a R151F transmission; other wise, if you have a G, W, or L tranny like most do, I would go with duals with stock 21 spline 2.28 gears in both cases with a Marlin Adapter OF COURSE, and later, when you are ready and want lower speeds, then buy a 23 spline 4.70 gear set and install it in the rear case.
The only issue with going immediately with the a 21 spline 4.70 gear set in a single case will be that YOU WILL eventually want to go with duals cases (because everyone can't stop wanting to mod his rig), and you will probably want to go with the 23 spline Marlin 4.70 gears in the rear rather than a 21 spline 4.70 set that you already bought.
My solution was that I saved for intially and bought a good conditioned-used R151F turbo tranny and went with a single case 23 spline 4.70 gear set, and I wheeled it for a season without drive lline mods; loved it but wanted lower gearing still so then I went with a Marlin Crawler dual ultimate (23 splines in both cases since they were behind a turbo tranny), and I did't have to worry about selling or doing anything with a 21 spline 4.70 gear set because I already had a 23 spline 4.70 gear set in my single case. I know 21 spliners work well (I am not rtying to knock them here), but I wanted max strength for a Toyota drivetrain, and I did not want my first single case mod decision to not account for later mods causing me to be wasteful later on. That's my two cents.
