Basically the flow is what you want in a radiator if the motor is making HP. A 2 row of 1" tube will flow better than 4 rows of 3/8. If you think about it, 2 rows of 3/8" is not at wide as 1 row of 1".
Yes......copper/brass does dissipate heat better, but there are other components in a copper/brass rad. Mainly the lead solder holding the steel tanks. There are more metals involved in a copper/brass rad, all dissipate heat at a different rate.
Aluminum rads work better with the wider tubes ,1. because they will flow better and 2. since the tubes are wider and the fins per inch are more, you have more air flow over them helping to remove heat.
There are aluminum rads that do have epoxied cores to the tanks...Griffin.....They do work well as I had one on my pro street car for 9 yrs with no problems. The downside is you can not weld on them. I always tell my customers to get an aluminum rad with all welded tanks. Then if you need to weld a tab somewhere, you can.
The one bad thing I will say about a wider tube aluminum rad is watching how much pressure you generate within your cooling system.
With wider tubes and too much internal pressure, you can actually balloon the tubes inside. What happens then is as one tube balloons it closes off the one on top and the one below it. If it happens to a few tubes, all of a sudden you have very little flow in the radiator itself. I had a customer who had a blown Willys with heating problems...........he brought the car to our shop and when we looked at the radiator, you could visibly see between the fins the tubes were balloned. It doesn't happen much but it does happen.