Charing Cross extension

It was in the last Horizon studies as a possible extension. If the DLR take it forward it would be developed after Dagenham (which is due to be open around 2016/17). There is a rival proposal to go to Kings Cross from the Horizon study. Time will tell but dont expect anything done till after 2012. All the DLR efforts are going into the Woolwich/Stratford International extensions and the 3 cars plans (with new rolling stock). Once the dust settles then they will start looking at stuff after Dagenham. (Work on Dagenham has already begun with the route choosen).

It depends on the nature of the service that you want/need to provide. Generally speaking:

LU runs higher capacity trains, at similar/higher frequencies
NR runs longer distance, often faster trains, with a lower density of stops and lower frequencies with a greater flexibility in terms of capacity.
DLR has more stops than LU and is cheaper to build and maintain than LU and NR as it is light rail rather than heavy rail.
Buses are the cheapest to run, have the highest density of stops and have the greatest flexibility of routes. However they are the slowest and lowest capacity option, and are the most likely to be disrupted
Trams fall between the DLR and buses. The DLR is essentially a reserved route tramway and I would be surprised if construction and maintenance costs for similar stretches DLR and off-street reserved-route tramway were vastly different, they also have a similar density of stops. Tramways are more expensive than buses to construct and maintain, but have a greater capacity (probably similar to the DLR, but lower than LU) and greater potential top speed. They probably have the greatest flexibility in stop density of all rail-based transport systems. I guess that on-street tramways through an urban area are cheaper to construct and maintain than something like the DLR if there are no existing alignments to use. Converting an existing alignment from heavy rail (LU/NR) to DLR or tramway would probably not be vastly different, excluding level crossings and singalling (both cheaper for the tramway).