I think what he's asking is which gives a smaller file size.
I don't think it really matters. That is to say, if you made all your triggers in GUI, and just converted them to JASS using Edit -> Convert to Custom Text, there would be no change in file size.
However, if you were to script in JASS as opposed to using GUI triggers, you would have full control over exactly how much script is written. You would be able to eliminate useless or necessary script that the GUI might throw in.
Still, as far as I know triggers only contribute to a minute amount of the map size. You should probably worry more about models and sounds.
(If this isn't what you're asking, just ignore that last paragraph )
The war3map.j file has an extremely low compression rate and ends up producing walls of kilobytes per system. These kilobytes are very important for users as it affects download speeds online, something that annoys many. Smaller file size is always better save if you're an already-downloaded resource like DotA.
That said, imported files are the best thing to avoid when publishing maps. Maps that load up on custom models had better produce something amazing for it to pay off.
You do know that GUI is automatically converted to JASS when the game starts, right?
So in the end, the only script that warcraft can read is JASS.
When GUI is converted, the script is massive. At least a lot bigger than what it could be when done in JASS from the beginning.
So if you just want a low file size, then you have to code in JASS.
However, Bribe is correct: importing files because they look good (instead of because they fit your map) is a negative point, I can even subtract points for that.
That wasn't really your question, but it does have to do with it ^^