Hi shirin,
I agree with Nick. They each have their own uses. WordPress ia a little easier if you don't have to do anything really special, and if you want to run blog, but there is quite a bit to learn if you try to do anything non-standard. It usually involves actually writing code.
I say this so much I know it sounds like a broken record, but the best one you can use is the one you know how to use. Just because it has a reputation as being easier, doesn't mean it won't be confusing and hard to work with. It is so different from Joomla, that you'll be almost back to the beginning of the learning curve.
Cheers,
Ed