Birch burr in my experience is pretty fabulous stuff, a very tight/ close grained pattern, and if left with some natural bark edge , contrasts nicely with the relatively pale wood . The small burr ( canker ) is something of nothing in comparison to the big one , I would sit for ages looking at that from different angles trying to decide the optimum way to get the best out of it , with minimal waste , but not compromising on the final form just to get the biggest I can out of it . Most of my work is hollow vessels and i can visualise a nice large one here with voids , bark inclusions , and possibly a natural edge opening , combined with the burr figure ,could be pretty spectacular . I would turn it whilst still green , as birch will soon start to crack and the bark could begin to separate as well especially in this nice weather . What ever you decide to do with it I am sure it will look great and every one will have their own opinions on what is best for it , and I am sure you will post the results !!!( incidentally how much would forum members value a lump such as this ?)