The easiest way to compare these two apps:
Locale costs more. But it is a lot more user friendly. You just set up "situations" with triggers and actions. Then you prioritize the situations. That's it.
Tasker costs less. But it is a lot more customizable. It's more like being given a scripting language. There are TONS of options and choices and it can get really confusing if you don't know what you are doing. It is not made for the average user.
I will say that anything Locale can do, Tasker can do it too. It just isn't as quick and easy to set it up.
In regards to backing up, you can backup Tasker into script files. Locale has no built in back up system.
Both of these applications are extendable to do practically anything because they support plugins. Tasker supports Locale plugins too.
Also: personally, I use both of these apps. I use locale for setting up common situations (disable sounds when asleep, adjust brightness/screen timeout when plugged in, go silent at work, etc.) and then Tasker for... well... tasks! Such as plugging in my headphones I reduce my media volume to 50% (I've blared music accidentally way too many times) and displaying a menu of music apps.