First, let me say that I think the BX23/2670 are mighty fine units.
They have a slightly larger footprint and a HP advantage over the BX1870. However, as already mentioned, that HP advantage means very little when the limiting factors are stability and traction. The 1870 already has more HP than the chassis can accommodate. Also, I can lift a heaping bucket of gravel into the back of our RTV with ease. Adding another 100 pounds of capacity to the loader would also seem of very little practical benefit. With a 54" mmm and the Kubota PTO powered bagger, I can cut and bag wet grass and leaves at a steady 3,300 rpm on any of our steep hills. Personally, I think the entire BX line is pretty darn awesome. We do have a 60 hp loader tractor as well. The little BX1870 has saved the bigger tractor many hours of operation.
If you have your heart set on armrests, the BX1870 may not be for you...
On Edit: Obviously, I don't have the 'bold' function figured out yet. The only word I wanted to be bold was 'slightly'!