Both files would be downloaded if I recall correctly.
Workshop is faster than FastDL because FastDL works on the principle that for each file it creates a new connection, then downloads then closes the connection. This can take seconds for a 1 byte file. The speed may be the same when downloading, but the constant dropping and reconnecting while downloading will cause it to be much slower.
On top of that, workshop addons are incredibly well compressed; 64mb in the workshop can be a huge file-size extracted.
Do a test; set up fastdl, clear your addons and try downloading. Record the time it takes from player_connect game-event until PlayerInitialSpawn ( actually, you won’t be in game by then, use this: https://dl.dropboxusercontent.com/u/26074909/tutoring/_zipped_code/acecool_hook_playerfullyconnected.rar )
Do the same thing with 1, 2, 3 or however many workshop files it takes to make up the data. You’ll see workshop is much faster.
On top of downloading going faster because repeated connections are made less frequently, when addons are extracted the loading time exponentially increases.
I had an issue running Lua files in M9k and other addons because it put me over the limit. I tried to extract all addons and delete the Lua files. Loading time was minutes when normally connecting to my dev is less than 30 seconds, if that. Not only that, but auto-refresh with the addons extracted literally locked gmod up for 45seconds or so. Typically with 3mb of code, no repeats, ~70,000 lines of code excluding whitespace, new lines and comments, it takes 5 seconds. With my custom auto-refresh it takes about .5-1 second.
Try that as a test too, load 3 gmas vs 3 extracted gmas and get the time. Compare it with the loose files.
I’m not trying to discourage you from creating the tool; I’m sure it will benefit many people, but please do some testing before deciding whether or not to proceed, it may not be worth it or you may even improve upon your own idea / solution.