25 degree rise stem vs 6 degree rise stem with taller head tube - how is handling affected? All,

I’m shopping for a new road bike with endurance geometry (more upright position and shorter top tube), 9mm drop saddle-to-bars.  I’ve had a Guru fit done by a professional fitter. The main problem is I’m 5’5” with 29” inseam, and one arm is 2” shorter than the other. So it’s been hard to find a small frame with a short enough reach and tall enough head tube (enough stack) to put the bars close enough without compromising handling.

Most of the 48cm or 49cm frames I’m looking at have a top tube that is too long and/or the head tube is not tall enough. Many 50cm frames don’t have enough standover clearance. The fitter has spec’d a 70mm 6 degree stem as a minimum and doesn’t want to go shorter. 

I thought it might work to use a 70mm stem with a 25 or 30 degree rise, to effectively reduce the reach and get the handlebars high enough. It’s sort of like making the head tube taller and the top tube shorter, but with a riser stem. However, my fitter has advised against this, saying that using a riser stem with that much rise may adversely affect handling and weight distribution.

My question is, how is using a stem with 25 degree rise different from having a frame with a taller head tube (more stack) using a 6 degree rise? Is there a handling difference, and why?

Thanks
The weight distribution is affected only by the location of saddle and handlebars related to the wheels. It does not matter whether it is by higher rise stem or higher head tube.

The difference that might affect handling is that system of frame with high head tube and short stem is stiffer than one of lower head tube and longer angled stem. You can feel the difference when pedaling hard, but any modern stem will be far stiffer than old-fashioned quill stems used to be.

There is a financial difference, is your fitter recommending custom frames?