OK, so then you have to ask yourself if the name is descriptive or not... as in, is the obvious use of that name this same service.
For example, If I want to sell wigs at wigbox.com and there's already a wigbox.net, it shouldn't matter, because obviously wigbox.com is probably going to sell wigs.
On the other hand, if you want to sell bagels at RoundUp.com and there's already a RoundUp.net selling bagels, people generally wouldn't think of RoundUp as a bagel place except for the fact that this other place is using that name to sell bagels (trademark).
That might help you decide.