If you want to put both radiators on the same level, use McMaster 4860K657 adapters then use the same barbs to gauge the restriction level. If even with the G1/4" barbs, the TC is better, it will be clear now.

I know G3/8" help with flow but this doesn't matter because we aren't using 5/8" barbs to take advantage of the extra size. 1/2" barbs and 3/8" barbs will have the same inner diameter no matter what is the threading size.