Can't say about phone specifically, but from an OS, go for 6.5. As 6.1 is more based on 5-6x (think non-phone pdas a la ipaq, axim, casio days). That pre-6.5 os was more designed for stylus input and less so for buttons / touch, so expect smaller controls. 6.5 moves towards a touch focused experience and for 7, expect it to be fully multi-touch friendly. From what I've done with 6.5 it seems a step in the right direction, definately better than 6.
Another consideration is the upgrade path to 7 when it does come. Nobody wants outdated software! Winmo 6.1 are mostly a no go for upgrade path and 6.5 is noted as a maybe- tbd by device.
Posted from my CrackBerry at wapforums.crackberry.com
Even most of the 6.5 devices don't really have the hardware specs to go to wm7 and even if they did, they will be fairly outdated by the time wm7 comes around.
