I'd go for m542 or m752. Various sources show that the gecko's perform very well, however the 251 only accepts up to 3.5A and 50V, yet costs about 25% more than an m752, or equivalent which accepts 70V and 5.2A. Running the motors on 70V as opposed to 50V is likely to make a greater difference to torque than a better control algorithm (if indeed that is the case) in the gecko, plus both the m542 and m752 are rated for sufficient current to run 3nm motors in bipolar parallel. That makes it nice and future proof as it's very likely you'll use 3nm motors on the next bigger machine.