I had this exact same problem when choosing the Heckler (which has same TT length) – I went for the large and shortened the Stem from 100mm to 70mm (+1″ TT length = -25.4mm at least on the stem length).
I choose the large option with a shorter stem and for me it was the right choice – I find the bike handles superbly and I have better control, the medium just felt ‘squashed’??
I would ask the LBS who has the demo to swap the stem for the shorter one so you can try it (probably easier than them getting a medium demo) – I can’t imagine they would say no, bearing in mind the value of the sale they might get… 8O
Just for your info here’s the geometry for the Blur in case you need it?