Percussion sidelocks shooting patched round balls are easiest to load, but I can't say that they are easier to clean. Having a removable breechplug like the inlines do makes cleaning more thorough and somewhat easier though, especially when you can see clear down the barrel like with a bolt action rifle. Otherwise the rifle breech never really gets cleaned to be shiney & bright like new ever again.
As for the 1 in 66" twist rate. While it is the proper twist for shooting .50 caliber patched round balls, that twist often requires heavier powder charges to obtain the velocity and RPM's needed for best round ball accuracy. That twist is usually more accurate at longer range and with hunting loads, but the 1 in 48" twist is also very accurate and doesn't require as much powder for shorter range shooting at say 50 - 60 yards. Plus it accomodates shooting conicals and some medium length & weight saboted bullets better, all while having a shorter barrel length in the 24 -28 inch range (instead of the 28 - 32 + inch barrel length of the 1 in 66" GPR and Green Mountain barrels).
For example, the Lyman Trade Gun and DeerSlayer have shorter barrels and 1 in 48 inch twists and shoot patched round balls very well along with the other projectiles. So do many Thompson Center sidelocks.
So every gun model and barrel twist has it's advantages and disadvantages.
If you don't want to mostly shoot stout loads, like for plinking at 50 yards or would like to shoot some conicals, then the 1 in 48" is a great twist to choose.
Then there's single and double triggers, barrel length, and whether you would prefer to choose a nipple/drum ignition or a patent breech. Again, every design has something good to offer.
For instance, the Lyman lock doesn't have a half cock notch with adequate room underneath for capping the nipple, requiring it to be manually lowered from full cock after capping to return it to safe. It's not a big deal but it's still a factor to consider when choosing it since most other guns don't usually have that problem.