Indeed you're right, you can do the same thing using pick nearest.
But still a circular sprite won't really be usefull here
Here is how it looks with pick nearest:
And if you know a bit how it works internally, this version is a bit less efficient than the one I provided.
And also, since the question was "how to compare distance between 2 instances", my code was merely an example of a specific use case.
You can do other interesting things like range checking or other fancy stuff (:Yann2013-05-23 19:13:29