Do both units allow recording of tracks(breadcrumbs)?
i'm surprised Garmin's website doesn't say (i just checked).
The 855 should because the x5 series do; at least that I've seen so far. I don't know about the 5000 may have to call Garmin
No need to call. I have a Nuvi 5000 and it records tracks. In fact, all Nuvi's in Garmin's current lineup record track. Only the older models (which may still be on sale some places) don't track - Nuvi 200, 300 and 600 series.
In fact, the new models are always recording track - you can't disable it. You can only clear the log and choose whether or not to show it on the screen.