Firstly, THANKYOU for asking BEFORE purchasing! Its no fun to have to burst someones bubble telling them they just wasted $600 on a glorified boat-anchor.
From what Breville say about the 820 on their site, 6910 every step of the way. Its in a totally different league to the Breville. For starters the 6910 has dual thermoblocks (one for brewing, one for milk) whereas the Breville only has one thermoblock that must be used for both purposes (nothing wrong with that in itself but its a lot slower than one for each). Like the Breville, the 6910 comes with dual wall / pressurized / crema enhancing baskets designed to make stale pre-ground supermarket coffee appear to give the same results as fresh coffee (which it doesnt anyway), but the 6910 also comes with regular baskets that allow you the best results from fresh coffee. The Breville doesnt have a standard commercial sized portafilter as the 6910 has either, so you cant just run out and buy regular basket in their place. The 6910 with its commercial sized portafilters means you can change baskets if you want (not that theres anything wrong with the factory single wall / unpressurized / real filter baskets), plus if you end up getting a better tamper for your 6910 theres every chance youll be able to use it if you upgrade your machine down the track.
There are a heap more reasons but Ill stop there. The only reason I can think of getting the Breville is if you liked the looks better. But if good quality coffee is anywhere near the top of your list, the Breville shouldnt be a consideration. For $100 more ($RRP) the 6910 is 10 times better. Frankly Im pretty disappointed at Breville - I believe they were trying to make a 6910 beater out of the 820 and had the opportunity to do some great things, but they appear to have simply added a couple of features to the already pedestrian BES800 and stay well and truly in the low-end home market with a price tag at the top end. Seriously whats the point of a pressure gauge if you just force the same pressure by using pressurised baskets? It says a lot about the whole machine - plenty of bling and shine but no substance.
Finally, there are plenty of 6910 users here who can help out if you have any problems. The same cant be said for the Breville - and thats no coincidence.