Ha! I wrote about this (badly) on my blog recently:
On the merits of non-hyphenation
In your case, you might want to consider what people search for. Do they type in "10000", or "ten thousand", or what? If you do these two searches on Google, you'll see that the results feature domain names containing those terms very prominently in each case. Try to find out which is the more popular version, the numerals or the words.