There is a little more CPU time, but the extended vertical blanking interval means there is a huge difference in video memory bandwidth, which is critical for a long DMA transfer from 68000-bus memory to VRAM through the Mega Drive's VDP.

60Hz V-blank DMA: 7524 50Hz V-blank DMA: 17622

That's at 320x224 resolution for both. I don't know how much this affects this one demo in particular, but for other things it can be a little annoying sometimes to be bandwidth-starved.