It all depends how accurate you want to be with your baro pressure in your AWS data.....I would be a little concerned that there is such a large discrepancy in the 2 values that you have, before deciding which one to use...If it is for your AWS then the best value will always be the Ordnance Survey value of ground level extracted from their maps (ie height above MSL to the OSGB(1936) Newlyn Datum)..then of course add the difference in height from your ground level to the baro device....Remember it is the datum that counts and there are a very large number of "datums" that the mapping fraternity the world over use (WGS84 being one of many, for positional info). Likewise, don't expect to go outside and use a handheld GPS to give outstanding altitude accuracy even if you have OS GB as the datum...relative poor GPS satellite positioning sometimes makes some quite significant errors in value...
Out of interest, my location altitude on Google Earth is some 20 odd feet in discrepancy to the OS ground level...I know which one I would trust..
Mind you Google Earth is an absolutely brilliant programme...you just need to realise the limitations....
Baggins 666 666