Or is it heavier? What other aspects could one consider here beside aesthetics?
I tested out my chromium install (Version 24.0.1311.0 (164559)), and it turns out that the GTK Version has a slightly higher memory footprint.
I tested it by enabling and disabling GTK Theme, while checking the memory footprint (Shift+Esc). I had killed all extensions to make sure they didn't affect the results. I also kept only the settings page open so as not to have webkit rendering affect it either.
My results were:
So, there is a difference, but its too little to be of any relevance.
Thnx to Capt.Nemo who showed me how to test this I made my own experiment, which should be seen as a complement to the his answer, that I'll set as definitive, in the sense that the differences are negligible: especially given that my results are different (GTK theme appears lightest)
Using Google Chrome 22.0.1229.94, also killed all extensions and let the help tab open
Aluminium theme from Chrome Web Store
(As a result, considering the "Classic theme" as a sort of "no theme" situation, and comparing the other two, I go for the GTK theme.)
Also, it is possible that the GTK theme would give different results depending on the desktop environment. My test is made in Lubuntu 12.04 (LXDE).