Re runtime - well, that's kinda par for the course with a built-in battery (and partly why I bought something that takes AAs)..if you had something with the capacity of some decent AAs it'd be bigger.
After quizzing bryton support about runtime, I got an interesting reply which said it depended on gps reception - ie in poor reception it'll take more power because it hunts for satellites. So leaving it on inside a building (ok, a bit pointless) or on a windowsill with poor coverage won't do the runtime any favors.
(It could, of course, be the same with Garmins..)
(Pretty sure I've posted this somewhere here on another thread..) the two main providers of gps mapping are navteq (a subsidiary of Nokia) and teleatlas (now owned by tomtom, but Garmin were trying to buy 'em at one point). They're both used on satnavs, so I'm not sure I'd get too hung up about one vs t'other. Any wrinkles are probably going to be going offroad where you might need OS-type coverage. The garmin city navigator maps are Navteq.