I use the free versions of both these tools, and realised the results are completely different. I am aware that one displays searched per month and the other per 12 months. However they have completely different results, even over the same time period?
This isn't unsual. These tools are not completely accurate however they are very useful for getting the trend of a keyword. Personally I prfer wordtracker. There also another that compares keywords across a number of different sites to get a better comparison http://tools.seobook.com/keyword-tools/seobook/
If I recall correctly, WordTracker pulls its keyword data from the meta crawler sites, and then on top of that only in a specific country (my paid account only lets me pull data from the US or UK for example). From what I've noticed, Trellian's KeywordDiscovery tool pulls data from a global database.
Keyword suggestion tools can give you idea about the keyword and it help us to target the page with relevant keyword. I am using google keyword suggestion, wordtracker and keyword discovery. I gather the data from these three and include the keywords in the phrases.