3underscore
I know at least one person on a TN who is desperate to change to an H1-B. The TN insists that you remain in the same job. Some firms will work with you and change all the responsibilities and everything under the same job title, but there are plenty horror stories about working under a TN.
Not to warn you off - I think the market for good international candidates is improving significantly.
It's been a while since I've had to deal with immigration issues, but how is the TN any worse than an H1-B? You can't change jobs with an H1-B unless the company files an "application to amend H1-B". The other good thing about the TN is that there is no time limit, unlike an H1-B where you get 6 years then you have leave the country for a year.
Obviously the best scenario is a company hires you under a work visa then immediately starts the green card process.
RF