cautionary issue is, can you turn the handlebars over the filled bag, and not hit, or snag
it with the brake levers?
The T bag is hanging from the top of the frame tube shoulder. and
the velcro straps on the bottom, pull it tight around the frame.
the frame is 30cm./11.5" tall S bag frame is shorter
(I dont have one to measure for you )
so, I expect the T bag will drape somewhat loosely around the shorter S frame.
how do you plan to make up the height difference?
perhaps get their S bag, frame included, and make up for the missing volume, with a big Carradice saddle bag..
for my size, the height of the M bar is right,
You, OTOH, consider looks more important.... OK
...