27" are not that easy at present, 700c which is about 4mm smaller in radius has a much more wide range of tires, and rims. I would go with 700c if you can and use a larger section tire, the bigger the air chanber the harder to "bottom out".

But then there is always 650B,,, kind of between 26" mtb size and 700c, typical cross section is 1.5" at the tire. Probably need a frame specific for them, Rivendell had a bike that used that size, many French touring bikes used the same size way back too, made sense for secondary roads (dirt) too.