A very interesting question here!
The 5951 uses the caliber first introduced in the 5959.
As said very thin and threfore with HORRIBLE tolerances for the watch makers!
But aside that it has got some much more interesting features:
- a continiously moving and not as usual, like in the 5204, a jumping minute display of 60(!) minutes and not only 30 minutes
- this is possible due to a minute colum that goes through THE ENTIRE clockwork, staring from the main plate up the the chrono element at the top
- this minute colum is also carrying the minute counter wheel which has NO CONTACT to the chrono second wheel !!!
- this all ensures less friction if the chrono is started
Therfore it is a FINE CHRONO!
BTW: the 5959 caliber at least is made out of German Silver and assembled twice, too.
I do not know if it is the same with the one used in the 5951.
Rubins, mainspring and hairspring are not from Patek, though.
Unfortunately it has not got an isolator mechanism, hence it is a "not so good" split second chrono, to put it mildly

!
Based on the 5959 the 5951 will have the pp calender part on top sitting.
The 5204 in contrast is thicker but a FINE SPLIT SECOND CHRONO witha pp calendar mechanism.
Fine as it has got an isolator mechanism, that is quite helpful to avoid wear and running.
Now, what you choose, what you see as a more desirable construction is something that you decide

!
Best
Moritz