I have used both Web Expressions AND Dreamweaver. I am also going to go out on a limb and just assume that the OP is NOT a dedicated/profressional web designer and that they may be planning on using the software to build their own personal website. If this isn't the case, I apologize:
* Web Expressions 2 can be attained for FREE if you are a student enrolled in an institution which participates in the MSDN exchange. Ask someone in your computer department about this. (This is especially nice since a LOT of FULLY LISCENSED COPIES can be attained for free this way, including operations systems and most of their development tools) There are also other legal ways of getting the software for free, but this is my recommended approach.
* Both Dreamweaver and Web Expressions 2 suffer from all the common problems that come with being WYSIWIG editors. The code that they create usually works, but not always, and users will SOMETIMES need to do their own footwork. Nevertheless, BOTH do their jobs equally well and can be used for most basic projects.
* Dreamweaver has a few sexy features; my personal favorite is its ability to download and upload my changes to my website on the fly, which completely cuts out my need for an ftp program or to visit my cPanel. It also has some intelligent project features that allow users to build larger websites that use multiple forms of programming/platforms.
* Web Expressions 2 is simple; it is easier to learn, cleaner, and just works better out of the box than Dreamweaver, especially for someone who's just learning to use the software or isn't planning on doing any major work. In fact, I personally prefer to use it to make tweaks, because Dreamweaver is a heavy.
* Web Expressions 2 loads and operates "better." When I say this, I mean it works better on my machines. This may not be the case for you -- if you care about this at all, I'd recommend doing a comparison. I use a laptop for most everything I do, I multitask, and I'm impatient, so the time it takes for a program to launch, load, and let me at my work, all while using fewer resources, is a consideration for me.
* Web Expressions 2 has a cleaner layout. I have found it to be more enjoyable to use than Dreamweaver on smaller screens; Adobe has this thing about sticking panels and extra windows all over the place and taking up valuable space.
* Dreamweaver is expensive, as has been mentioned previously. One can always pirate, but then, one can always get fined by a multimillion-dollar corporation. ((Come on guys, illegal things are illegal.
))
* Dreamweaver works well with its "sister" software suites from Adobe. If you use Adobe products often, and you're accustomed to them, then maybe you will enjoy using Dreamweaver more than Web Expressions. Web Expressions may work well with its own "related" software, but I haven't tried it and can't say anything for it.