Daylight simulation newbie

Hello

Im totaly new to light simulation and I just got a very general newbie question. At our office we have descided to give simlation a try. Now im not just talking abou shadow diagrams and so on, but calculating reliable data on daylight levels in interiors aswell as exteriors.

After a quick look around it seems that daysim and radiance are two quite popular programs. First question is: is Diva for rhino the same as daysim just implemented as a rhino plugin? 

The second question is quite open to anyone who knows what they are talking about: what are the main differences between radiance and diva for rhino/daysim, and how are they when it comes to using them in grasshopper?

thirdly: are there any other good rhino/grasshopper daylight simulation tools?

any kind of helpfull tips or information is greatly appreciated!

-Martin