The current Sturmey Archer five speed is a sweet running unit that we have used several times with no problems.
 

 
The bike above belongs to regular customer Joe Feeney. Two years of daily use after its last overhaul it came back in and had a SA 5 speed with coaster brake fitted.
This is a good, reliable and versatile bike. This kind of machine would cost between £300 and £500 depending on detail specification.
 
To convert your existing bike to 5 speed would be from £150.
 
The Sram P5 is more expensive and probably better.
 
The old S5/2 was an interesting unit that had dual cable control. The right lever worked three gears, and the left converted the hub from a wide to a close ratio. This made the shift sequence very interesting. Once you got the hang of it, it was fine, but the target market of people who wanted something simple were confused and it was replaced by a single cable version.