They are both excellent scales and you will never need another scale if you buy either one.
I bought the 1010 because I wanted to be able to weigh things over 500 grains and because it "self stores" in its own case. Also, I preferred the drum poise over the sliding poises on the 505.
If you can throw a plastic cover over the scale in a reserved location (and so don't see an advantage in the self-storing case) and if you don't mind moving little poises around, the 505 is fine.
The 505 is a little cheaper, so if economy is a consideration, there is that, too.