When you live in a "new" build house the windows are pretty much the same sizes.
ie on my house I now have 4 of the 3/3/3 windows, 3 of the 2/2 windows and then 3 singles (lounge, bathroom, en-suite, I have a french door bay window in the lounge too.)
Anything that varies from that does not blend in and screams garage conversion.
The 50/50 split wouldn't work because the new windows and the upstairs window are not the same size.
When you live on an estate like this and the houses are similar designs any new stuff stands out unless it is done well. For me I would want the 3/3/3 if that would fit.
I do agree with the decorative top not having enough room due to the original header bricks from when it was a garage door.
Your house is very similar to mine, in fact I could guess that the downstairs room is your lounge (mine is a dining room) but your upstairs is possibly the same, so master bedroom, en-suite and bedroom 4? The only difference is my dining room had the 3/3/3 as did the master.