Hi there,
Try out both... I must admit since I posted, I do now use the "SEO Competition" feature on Market Samurai...
Reason is that, that produces is a nice one page summary that makes deciding on which keyword will be easy or hard to have a chance on ranking for e.g. shows # backlinks, domain PR etc.)
This summary on one page for seo comparision with top 10, is missing in link-assistant tools unfortunately (and I hope they add it.... THIS IS A HINT LINK ASSISTANT STAFF - as this was an add-on that some people suggested goes into web-auditor, but perhaps would fit better into ranktracker, as it is a sorting out keyword ranking ease table, which more closely relates to the keyword research exercise, which is presently done in rank-tracker with link-assistant tools... the MS version could even be improved upon and averages for top 10 for page Domain age, Page PR, lowest # BL page, higest # BL page and count data for onpage attributes i.e. a nice summary for each keyword that goes into keyword research KEI page of rank tracker

).
I however use link-assistant tools for search volume and competition data - as prefer to work on accurate data.
Also tools like seo-spyglass, webauditor, link-assistant and there features do not appear in Market Samurai.
Rank tracker, has additional features compared to that module in Market Samurai and does calculate rankings for country / market / search engine (i.e. can be set to work with locale).
Recently Market Samurai have added another couple of modules, which can be useful for link building or affiliate sorting out...
Really the tools are perhaps going different directions at the moment and their is use in both of them now.
However I would still prefer link-assistant tools over Market Samurai at the moment if I had to choose between them.
So perhaps download the free trials (test accuracy of results manually), watch the videos and decide for yourself...
Hope comments help

- cheers, Asiaplay