I go for painting first, and that is mainly based on personal preference in style, so not very objective - but I count on statistics to dilute my vote and while ocasionally I find with some surprise that the one I voted for and found to be "better" is trailing by an hefty percentage in the tally once the results appear (after voting).
If it looks to me like it's more or less the same painting quality, I go for my gut instinct. "Something" that makes me prefer one entry over the other. And that goes straight to personal preference in theme, composition, humour, etc.
I (as others) might be biased subconsciously against GW, hopefully not "just because it's GW", but because it's limited in theme (it's basically either WHFB or W40K) and a bit "too" familiar - in other words, my imagination will easily be captured by a Steam powered automaton wrecking havoc on a zombie horde and less so by a squad of marines.
But this thought process is the tie-breaker. The competition being clearly stated as a PAINTING competition, I go for technique first (albeit, as stated, based on personal preference), and all other factors second, and I have more than once voted on GW entries without flinching.
That being said, I think a mono-manufacturer round would go against the grain in the spirit of the competition, especially (but not only because) it will automatically limit the theme and therefore will be more "boring" to watch
