ebisoba, you mention you're not upgrading in the near future and you run 5.5. Questions would be, what is your comfort level with 5.5? Have you been administering it for quite awhile and had your head in admin guides and redbooks and have a good understanding and have troubleshooted, etc? If you have, you may want to take 6.1 class, but again you're not using 6.1. If you're new to TSM and you run 5.5, I'd say take the 5.5 course to understand one for one what your working with daily.
If it looks like you'd have the opportunity to take the 6.1 course later, take the 5.5. If this is one shot at it and you won't have another, think about your needs based on what I've mentioned above.
Just some things to think about. Not a definitive answer, but there really isn't one from here. You know your knowledge level, your company, your needs.