The GH-625XT tracks the position on the map more accurately than a GF-305.
(I have compared the devices simultaneously on a number of occasions.)
The hardware in the 625XT is very good.
Instant pace is occasionally a little more off in the 625XT. I would describe it as the averaging algorithms differs a little. I do not see this as a problem, GPS will never show the instant pace perfectly, I know the pace better myself and use the average since last lap. The average for last kilometer in the GH-625M beta firmware was a really useful feature.
In my view, the distance calculation is a bigger issue. The device calculation works similar to a friend's GF-310XT, which overestimates distances. For post-processing (for instance in SportTracks), the situation is similar, at least for GH-625XT to GF-305.
This should be able to be addressed in the firmware. Globalsat has supported the 625M firmware for a long time.