I have both a Gold 10 and a HK SBE. Both are great guns and both get the job done. It all depends on what kind of goose hunting you are talking about. Over decoys, the shots always tend to be close (under 35 yards) which the 12ga does fine. If you are pass shooting, its posslble to get 60 plus yard shots. Then, thats where the 10ga really comes on strong. This is for two reasons, bore size and better patterns plus shot density is larger with its ability to handle bigger pellets well. When I got my first 10ga, I could hardly wait to get one and I was only using it for ducks at that time. It did offer better killing power which allowed me much more effective shots at the occasional passing goose that was offered. When things are normal and I am doing a combo hunt for both ducks and geese from a permanant blind, I have both guns loaded and ready to rock and roll in front of me. Using factory ammo when I got my first 10ga, I had more range with the ten for whatever I was shooting at. At that time, I was shooting a Win load which was 1 3/4oz of #1s at 1260fps. At that time, it was much more awsome than any 3.5" 12ga load.
I know this is not much help, but if I had to give one of those guns up, it would be a hard decision. They both have their advantages. For most shooters, I would say the 12ga would be the best bet. For me, I most likely would choose the 10ga.
Also, if you have an outstanding reload recipe that makes a consistant patterning shell, that makes the 12ga an awsome gun to shoot. Last season, my 10ga stayed home and I only shot my SBE. The load I chose was a handload which was 2 3/4" with 1 1/8oz of #2 steel shot. This load is outstanding among all the rest of my reload recipes and does a fine job on big geese out to slightly over 50 yards. I measured 27 kills all from 48 to 54 yards with this load with a laser rangefinder. I could not have expected any better results had I been shooting a 10ga. If those shots had been 60 yards though, it would have fell short and the 10ga would have shined. I might add that I tend to like the older SBE much better than the SBEII. Basing your decision on my opinions is not a good idea. Pick the gun you like the best, period. The one you like best will most likely perform the best for you. The way a gun feels and swings is so important. When I handled a SBE for the first time, years ago, I knew it was the gun for me. That has not changed. The SBE II doesnt do it for me but its still a super gun for some.