The ONE is much better then the 300..
1. The GPS Chipset is the latest Sirf III, this locks extremely quickly, is much more accurate in marginal reception areas and will lock even through a athermic windscreen..
2. The Processor is much faster, the 3D display fluidity is excellent, as is route calculation and re-routing..
3. The ONE has the same screen size, but has a smaller form factor, it is just about pocketable..
As for speaker clarity, the ONE is extremely good anyway, in an old diesel Veccie, cruising at 90, it is extremely loud and clear.. I've never heard of a single ONE owner saying the speaker is not loud or clear enough.
The Battery does not last as long as the GO300, this is because the unit is just about pocketable, unlike the GO.. but it comes with a car charger, and lasts for 2-4 hours which is long enough for most journeys should you forget the charger...
Since it is the same price as the ONE, unless you absolutely need the slightly longer, but still not stellar battery life, you'd be mad not to get the ONE..