Myself if you have the choice , go with a 54 over the 50 . 
 Rifling , round bottom . IMO experience it fouls a lot less , cleans easier and in most cases will group a round ball much better then folks can shoot .
As to if  to go  with a match grade barrel or not . IMO a good barrel is a good barrel . You purchase one from a quality   maker and you wont be sorry 
 Match grade means the accepted tolerances are much closer   and more consistent  normally this  depending on maker can be found to be no less the .0005 down to .0002 
 Also the bore will be highly polished  so has to have not even  light scratches either in the lands or groves . .
 Does this make a difference ? Ya it can  if your shooting for POI that is measured in 32ths  or long distance shooting 
 Past that , if your going to be using this gun for hunting or  trail walk / standard rendezvous shooting .
My take is there is no need for this .
 I think to often  with muzzleloading barrels match grade  is really nothing more then a barrel that has been lapped  and cleaned up so as to ensure no sharp edges .
 Back in the old days  it also meant no run out  but with today’s tooling , that’s really minimized anyway .
 I would seriously doubt that  if you compared a matched grade barrel and a standard barrel of proper quality and make , that’s had a good load worked up  through it ,  the average person would see any real difference.
 Now I know some will disagree with that  but the proof is in the pudding .  When you get you clock cleaned on the range by a fell shooting an old CVA with a Jukar barrel   and your standing there like penny waiting for change with a high end custom gun  with a 250-400 match grade barrel .
 You have to ask if the extra 200 was worth it .
 but if at the same time , a near perfict ,  barrel that needs little to no break in is what your after then match grade is what you want