I'm a Harmony One user and have been since the remote came out. It's my second Harmony remote (I have a 670 that was retired to the bedroom), and I've never had any complaints. I can understand complaints that certain power users might have with Harmony products if they need extreme customization of multiple sources, but for 99% of what I'm trying to do at home (e.g. Watch TV, Watch a movie, Play a Game, Listen to Music) the One works just fine, holds a pretty good charge, and almost never has problems turning on/off components and getting settings right.
As somebody else mentioned earlier (Josten?), I would just go ahead and buy both, particularly since you have the credit from Harmony, and either return/sell the one you don't like.