I use both GL6 and DW MX. Background - GoLive was the first "real" wysiwyg editor I used (actually, PageMill was the first, but it was for home use, not pro use). I used GoLive right up until version 4 came out. Right around that time, I took a new job that used DW. It only took me a weekend to learn, and I really grew to love it.
As they both stand now, they both are very capable editors, and I'll run down each one's strengths/weaknesses...
1) Interface - Both interfaces are well done. DWMX uses the new palettes that Macromedia has standardized on, which makes tidying up quite easy. GL6 has a new feature where you can save your workspace, and name it, allowing you to quickly get a certain palette arrangement back.
2) Site Management - Dreamweaver kicks GL's butt here. First of all, GL creates sites "files", which DW doesn't have to do. Also, DW has the site window, which allows you to quickly scan through all your sites and select which one you want to work on. GL treats sites very differently. There is no way to browse through all of your created sites - you have the most recently worked on sites in your "Recent Sites" menu item, but that only holds a couple. DW's implementation is just so much better than GL.
3) Rollovers - GL6 has a much easier means to implementing rollovers. Each image tab has a rollover tab as well, allowing you to easily set the over and down states quickly. DW has a rollover button that can quickly set up rollovers, but if you already have images set and you want to add rollover effects, you have to add a js behavior to it, which is a bit tricky for first time users. Setting up swap image rollovers are easier in DW.
4) CSS - DW has a much more simplified means of implementing CSS. You can do everything from one pallette. In GL6, you have two pallettes to select and edit/define CSS, and another to set properties. No contest - CSS is much easier in DW.
Overall, both are great editors. I'm probably more biased towards DWMX because I've used it longer. However, GL6 has some great features that DWMX doesn't have. GL6's Site Diagraming feature is great. You can create entire site maps in GL6, something that most pros would do in Visio (or ConceptDraw on the Mac), output them to PDFs, and when finished, use them to automatically set up your site structure. It's a great tool that has no equivalent under DWMX.
Another pro for GL6 is the price. If you own GL3,4, or 5, you can get GL6 for $99. I bought a copy of GL4 on eBay for $25, and upgraded for $99. DWMX costs $199 for it's upgrade, and $399 otherwise. Basically, it's $100 more than GL for both the upgrade and the full version.
Another thing kind of annoys me is that I own the DW/FW3 Studio. However, to upgrade the entire studio to the new DWMX Studio, it's going to cost me $399. Granted, I'll get FlashMX in that upgrade as well, but I'd rather space my purchases out. Plus, it comes with Freehand10, which I already own.
You can download DWMX and give it a whirl for 30 days. That's the best way to figure out if it's right for you or not...
For me, once it comes out, I'll probably use GL just for it's site diagraming features...