Following Ryan Holiday's recommendation (http://www.ryanholiday.net/reading-list/), I highly, highly advise you to go with the Gregory Hays' translation (http://amzn.to/XQSimT)!

I have a copy of Long's and it doesn't do it justice.

If you want to dive deeper consider Pierre Hadot's 'The Inner Citadel: The Meditations of Marcus Aurelius" - http://amzn.to/YuWyvh

I am also fond of these YouTube lectures on the Meditations: http://youtu.be/nLD09Qa3kMk