Kind of depends on you and what you are comfortable with. If you are tall with long arms you might prefer a longer stock. I am average height with shorter arms (32" sleeve) so for me I always go short. I have the short B&T and find it really comfortable.
Also, I just feel like if you are going through all the hassle of making an SBR, might as well go as short as you reasonably can.
I wouldn't agonize over it too much, if you don't like the one you bought you could always sell it and recoup most of you money and get the other.