LIke Aaron states, both should work.
However, I'd even go one step further.
Make your own ULX addons modules folder.
Editing 'our' fun.lua file, your changes would be overwritten (or file not updated, depending on your SVN installation) when you next update ULX.
Make a folder in addons called "MyULXStuff" or something like that. (Any name, as long as it doesn't match an existing gmod folder, would work)
Within that folder, create a folder structure "/lua/ulx/modules/sh" (So, if you used myulxstuff, it would look like "/garrysmod/addons/myulxstuff/lua/ulx/modules/sh"
Create your .lua file in that sh folder.
Copy an info.txt from gmod/addons/ulx to your /addons/myulxstuff folder.
Edit the .txt to match your author/credits, description, time and date etc.
Something like below should work as the info.txt
"AddonInfo"
{
"name" "MyULXStuff"
"version" "1.0"
"up_date" "02/23/12"
"author_name" "Digital Spit, Ulysses Community"
"author_url" "http://www.ulyssesmod.net/"
"info" "My ULX Module addition hacks"
"override" "0"
}
Now, you can update ULX without worry of it breaking your own edits.
You can develop your own ULX commands and add to your own 'addon' area without having to edit our files.