You don't typically get to pick both tube size AND material. Larger tubes are almost always aluminum (stronger material), while the smaller tubes are typically the copper/brass. Aside from just comparing tube size, you need to look at tube spacing and number of tubes. At the end of the day, you can get a lot of heat to the radiator, but if the airflow isn't there, the heat doesn't come out (More tubes and less fin = more pressure drop on the air side for the fan to overcome).

Pick a product from a proven, quality supplier (Glen Ray, Griffin, etc) and you'll be fine. It all comes down to the look you want in the car too.