Firslty i was a crewman commander, on both chieftain and challengers, the challenger while yes as with any tank has its drawbacks, however the tank is well battle proven.
The L30 / CHARM3 is one of the best tank guns in the world, as for the split ammo system, it is a very safe, the rounds are all KE, the charges are stored below the turrent ring in cooled pods which self seal and designed to contain any fire, as well as the tank having a self activated fire surpression system,
the M1 design has not changed much since the A1 version, its slower and heavier than the basic model and the gun changed to the german 120mm, challenger 2 is a complete new tank, ATDU are currently seeing if the german gun will be any good on challenger but so far we are not convinced, as the tank would require some major internal redesign, the gun is not as accurate and also would reduce the number of rounds carried, if cased ammo, as an example chieftain carried 64 rounds. challenger 2 currently carrys 52 rounds, if cased the tank may lose ten or more rounds in the redesign.( mind you depends if we will have any after the defence review)
As for loading, during the cat cup in germany, both chieftain as well as challenger 1 crews managed three rounds for every one M1 round fired on range 9 a hohne ranges. Where the americans were all instructors from the armour school, the british crews were infact the actual tanks crews, the british crews on the ranges were only beaten by a canadian team in a leopard call me bias but i would be happy to pitch myself against any american gunnery crew in an M1. chieftain crews even managed a 8 round per min on Lulworth ranges in Dorset.
Americain tankers have offten considered challenger to be the more reliable of the two in the field.