Cheno, just on the off chance that an argument about why "this tractor is twice as much tractor as that tractor", may not help you very much, may I make the following suggestions?
First, making a judgement about which one is better for you depends mostly on what you plan to be doing with it, the condition of the units - after all they are used tractors, and what they are selling for.
The 4200 is a larger framed unit that would be ideal in working on a 2-10 acre rural property with mowing, groundskeeping, road maintenance and light ag chores, to name a few. It could also serve as a small chore tractor for farm use, I suppose.
The 23 is about the smallest rig you could get that is still more a tractor than a garden tractor (although some who don't own one would argue the point). It's lower, has smaller tires, shorter, narrower, etc. It will go into places you couldn't fit a larger unit. It is probably ideal for 1-5 acre properties, or places where smaller size, lighter weight and lower ground clearance are an advantage. It's easier to transport. Maybe most different is a
BX23 comes with a backhoe that would be an add-on with the 4200.
You haven't told us anything about what you need a tractor for, so the advice you get about which type of tractor is best really doesn't mean very much to your situation.
By the way, both JD and Kubota make superb tractors in the "compact" size (i.e. the 4200) and the "subcompact" (i.e.the
BX23) size ranges.