That's completely variable, depending on gears and tire size. I'd avoid the 4.10 gears for anything but heavy towing - that's what they are intended for and become a drawback in terms of fuel mileage and streetability under most other circumstances.
I've owned the '07 w/48RE for almost two years now and not a single complaint. Only issue was getting a solenoid changed out of the trans at 25k under warranty, but that was a minor, common issue due a bad run of solenoids, nothing unusual or inherently design suspect. 35k on the clock now and seems like it'll run forever. Maintenance is as big a factor in reliability as the equipment itself, regardless of brand.
The stock intake is fine for at least 500hp on the late model Rams.
The 48RE is a time-tested virtually bulletproof gearbox. The main reasons people have any problem is beating on them with boosted lauches at 500hp and lack of maintenance. Duh. If you're going to do that get a purpose-built tranny and change the fluid once in a awhile.
Again, gears and tires will determine your realtive RPMs. It's a safe bet you'll be happier with the automatic for a mostly daily driver-type truck. People have put on millions of miles towing with the autos and personally, I don't find plowing much fun with a manual, although I believe the stick will probably hold up better over the long run in that application if you are plowing alot.

 

who told u u cant plow snow with a cummins? cause thats bull my cummins loves it. the only problem is not enough weight in the *** end but jus throw somethin in er and go to town. im kinda biased on the auto or manual. i got an auto and its been nothin but trouble but then again its a 99 with teh 47re and i have heard the new 48s are a lot better ull be happy with which ever u decide