Hi Dj,
I only know of one member that has legitimate experience with both those detectors. I haven't used a 900, but I've been using a Legend exclusively since it was released.
Both those detectors have similar features, but the Legend does have a real time ferrous / nonferrous ratio meter (Ferrocheck). The 900 does not have anything like that. IMO, Ferrocheck is an invaluable feature when hunting in iron. Now that I've learned how to use such a feature, I would not want to use a detector that didn't have that feature, when hunting in my iron infested sites.
The raw performance of each will be similar, but the Legend is also much more affordable, and the coils are lower cost as well. The only negative aspect I've heard about the 900, is its erratic TID.
There is a very knowledgeable and objective member on this forum named jmaclen. I respect his opinion a great deal. He has owned both, and still uses a Legend among other detectors. He sold his 900. I don't want to speak for him, but I know he wasn't very happy with his 900. Hopefully he'll see your post and chime in with some elaboartion.
@jmaclen