What are the pro's and cons of longer boom length and shorter luff and vice versa for a given sail size?
Eg:
Brand A, Freeride sail 6.6m, 456 luff, 204 boom.
Brand B, Freeride sail 6.6m, 464 luff, 188 boom.
Ask a sail designer ? Good Question I have always wonder myself but with all the r&r testing going on I would say the shorter boom length would be the better sail easier to jibe or tack 
A sail with a shorter boom length can actually be wider than the other sail of same size.
Had a Reflex 11 5.6m that was extremely short luff and wide design,but had a massive boom cutout.
Short luff design, i think have more bottom end,downside is it's more difficult for the sail to spill the wind.
I've read the suggested link, This is a quite complicated topic. There are too many variables and considerations. In a nutshell, it looks like it's not a good idea to compare sails just based on these two measurements.
yep. that's it in a nut shell as you also need to know specifcation like draft depth/position and twist. in 2 sails of exact same dimensions if you vary those 2 elements the sails would feel and handle quite diferently.
but there are very general rules of thumb mentioned at the start of the thread.
