The difference is that Option 2 short circuits all of the radiators and will be the path of least resistance - meaning water will always try and flow through that path rather than where you want it to go (i.e. through the radiators).
Ok thank you
It makes me wonder why we bother using 22mm at all though, if it's going to be bottlenecked by 15mm at the end of the run, why don't we just do the whole thing in 15mm?