I know I'm a bit biased, being from SEOmoz and all, but I would say that very honestly, you need to figure out what you actually need. SEOmoz provides SEO training and tips content, some tools for link analysis, competitive analysis and link building and a Q+A section to ask questions. Wordtracker provides a very specific keyword research tool to help you discover and target new terms around your primary topics/subjects. It's like choosing between apples and a shirt - they're not really as comparable as you'd think. If you need a good KW research resource, get Wordtracker (or KeywordDiscovery which is also good). If you're seeking more broad SEO materials and tools, go for SEOmoz (or one of our competitors like SEOBook.com). One nice thing - I believe Wordtracker has a 30-day money back guarantee if you're not happy with it, and SEOmoz does as well, so if you find you've made a mistake with the service and want something else, you can always use that avenue. Best of luck!
I've used the Wordtracker free trial, and I honestly thought it was terrible. I was looking for something that SEOBook.com offers for free, and wordtracker was nothing like it. Maybe it was the niche I was searching, but the keywords wordtracker was spitting out for me were rubbish. I consider keyword discovery very good and much better than wordtracker.