IMHO, in the field of ancient coin collecting, it really doesn't matter which grading service one uses if they choose to have their coins graded and slabbed. All three of these companies are well known and reputable, however, the vast majority of ancient collectors do it for the pure enjoyment and not as an investment; as a numismatic study; as a historical journey through time; and so coin slabbing isn't really necessary for most of us anyways.
Also, I think most would agree, that handling and studying our coins on a regular basis is part of the allure of the hobby, and as such, many collectors frown upon the idea of slabbing an ancient coin. After all, the coins lasted 2000 years already. I am sure they will be fine for now without being entombed in plastic.
As a comparison, consider a Morgan dollar
. A completely different scenario. Go to any local auction and you will see slabbed Morgans. For every purist collector of Morgan dollars
, there is likely two collectors (or investors) who are just accumulating collectable bullion silver as a investment hedge in their portfolio. As such, grading and slabbing is necessary for them to realize their investment objectives.
Having said all this, I realize that slabbing has a place in the high octane world of ancient collecting where big players are paying tens of thousands of dollars for premium grade rare specimens. In this case, a particular grading service may have preference over another for reasons that don't have anything to do with why most of us collect ancients.
Anyways, welcome to the hobby. There are many members here that are far more knowledgeable than I that are always happy to answer questions or share their expertise on a particular subject.