I would have gone the city route as well, since there is so much support for it (quarries and the various +buys) that the city pile should be empty in no time flat, and that's really all you need to do since we have lots of +buy already - platinum/mint would be icing on the cake once you empty the city pile. However, if there was no support for cities, you would probably be correct in avoiding them entirely, although I still don't think trader is correct here either way.
decent game-plan:
1) open quarry/develop
2) get a worker's village and possibly another quarry, while developing estates into woodcutters
3) get as many cities as possible. If the cities are running low, you may wish to develop a quarry into city/$3card(another develop perhaps)
4) Once the cities are gone, get a platinum
5) At the end of your next turn, play all of your coppers, then buy a mint (or if you severely won the city split, you can avoid buying mint altogether and instead gain it mid-turn by developing a now-useless $4 card).
6) mint platinum and buy colonies, all while using the develops to get rid of useless cards like copper
note: you may also consider doing Level 3 (or 2, for you programmers) cities by buying out a 2nd pile, depending on how the city split works out - your opponent eventually recognized this and that's why he bought out the WV's
I make no claims about this plan being entirely optimal, but it's a start, and it should be faster than 'province rush with trader' (and it's more or less what sharky was saying). It's also a bit more coherent than what your opponent was doing. In general, trader is not so good in colony games (unless it's a curse fest or something).
finally to answer your question: you were rushing provinces, so you shouldn't be worried about buying colonies! so I would say get like 1 gold, then only buy provinces (like on turn 8-9 instead of gold), then you would have had more of a chance at ending the game while ahead. Also I would have trader-ed the develop on turn 10, instead of developing the trader into a city which helps him. Cheers