Why not argue the i5 runs cooler and therefore is the better choice, how is this argument different from your Fusion drive argument? The 256 GB SSD is larger and faster compared to a 128 GB just like the i7 is compared to an i5, no question about it.
BTW, any two drives can be fused, if you like mixing your boot partition with a slow HDD and increasing the chance of data loss.
Good point. However, I think the automatic management on 128Gb vs manual (and far lest space-efficient) management on 256Gb is key here, as I claim the former is better than the latter and you vice versa. It's much harder to compare than, say, temperature of an i5 or power usage or speed.