BFW can achieve low RPM but is limited at 6000 RPM. Kress can achieve 29000 RPM but it cannot go lower then 10000 RPM (5000 RPM on their slightly more expensive 1050 FME-1).
BFW is quite silent as it won't reach a high RPM. Kress is ok at its minimum 10000 RPM but it gets noisy quickly as you bump up at the RPM.
BFW torque is quite low, it doesn't take much to stall the motor - that doesn't mean you can't successfully take _very_ shallow passes in aluminium and soft steel to complete your job. You might hear Kress slightly slowing down during heavy cuts, but it won't stall - not a good thing either, if your feedrate was planned for a specific RPM.
BFW will come with up to 6mm collets only, while you can get up to 10mm collets for Kress, both proprietary format, not matching something like the ER standard.
BFW is slightly lighter then the Kress, but not much. They are both 43mm collar.

Overall I would definitely go for Kress for a CNC machine, BFW is more of a toy and it will work better with a toy mill like the KT150 combo purposed by Proxxon. However there are better choices in the price range, such as the 0.8kW chinese spindles.