Calendar Ext vs. Portable Sunbird

I was wondering what the Differences were between Portable Sunbird and the Calendar Extension.

Just major differences in a list is fine I dont need a review of each.

On a side note, the Calendar Extension wont install in PFF 1.5, It says the compatibility is 1.0+

edit: Upon Further research, I found the answer I was looking for, and also found that the 1.5 problem has yet to be addressed by the developers.