Im a GF owner so have some bias.
IMHO the BM has been around that much longer to establish a following and that following have tested and help refine the current product on the shelf. So there is much more in the way of BM taylored recipes and tricks provided by the brewing community
The GF is just over 1 year old on our shores in the UK. So has less in the way of user provided tips and tailored recipes..
The BM is made from thicker sheet stainless steel, has a cyclic bottom up recirculation , and a more sophisticated controller.
The GF comes as a more complete package with hop filter and cfc chiller as part of the bundle but has a thinner walled pot, recirculates the liquor through the grain from the top down using gravity. But has a simple controller requiring more input and external timing.
MY #1 concern prior to brewing with the GF was the probability of the recirculated mash liquor just returning to the pot bottom via the overflow drain and not through the grain bed, In practice after the initial 10-15 mins of the recirculation during the mash the liquor stops finding its way down the central overflow and feeds through the grain bed, and was crystal clear too
Without having had any BM hands on experience my endorsement of the GF may nor carru as much weight as others but it gets it regardless , Yah GrainFarther woohoo!!..