I've been using Nirvana for a long time as my only tool to support my GTD system.
I like it because it's nicely done, coherent with GTD, efficient.
By the way 2 weeks ago I've switched to Wunderlist.
For ONE reason : MOBILITY.
Wunderlist has an ipad and an android appplications.
These applications are nicely done and allow me to be 100% sure to access my actions even if I'm not connected to a good network (which is quite often finally).
If I want to have an efficient system it must be accessible on my ipad and or my galaxy tab, everyday, in all locations, online and offline.
For this, a dedicated application is a must.