Flips all the way (or soon to be, at least). You just cannot fit enough information on a cardboard holder.
Some of them don't fit in cardboard or flips I have a Ptolemy IV from Egypt that is an AE46, it is a monster that has a holder unto itself.
I use acid-free paper envelopes... I'm old school. Actually, I use these because the ancient dealer that got me interested in ancient coins suggested it as the best way to store ancients long-term. I've had some coins in them for 3 or 4 years now and they're perfect.
I too have a fondness for acid-free paper envelopes -- I have some Roman Imperial coins that have been stored in them for over sixty years with no ill effects. Having said that, I have been primarily storing my coins in unplasticized safety flips for many years now -- again with no ill effects. When I first started collecting ancient coins acid-free paper envelopes were all that was available and that is what everyone used -- with no problems that I remember. I don't think that plastic flips came into use until the 1950s -- then it seemed like most collectors turned to them by choice. I don't remember when folded and stapled 2x2 cardboards came into use -- I remember using them for awhile but I didn't like them. James
I was just told that the PVC in flips will destroy my coins and not to use them , ever . Bobby :bigeyes:
you need to get the non pvc flips. they are a little stiffer and some are brittle but its worth it for the proper protection.
I actually keep all my coins, ancient or not, in PVC free flips. I like to be able to easily take my coins out and look at them unobstructed, and you can't beat the room for information. In addition, the stiffness of PVC free flips allows you to easily stand them up for display. I have an AE40 of Ptolemy II that I keep in a flip despite the fact that it's practically bursting out of it. I feel that the tight fit hilights just how huge the coin is
Neither. I prefer open trays. Almost all ancient coins have been cleaned to some degree so anything air tight can hold in gasses that should escape. Second choice is paper envelopes but ancients are made to be handled and trays makes that easier.
As I mentioned previously, I am a long time user of acid free white paper coin envelopes for storing my Roman Imperial coins. I have some coins I have stored in this manner for almost sixty five years with no problems whatsoever. However, a couple of years ago I changed my storage (and coin transportation) system to that shown in the attached photo. I made the change for three reasons: 1. I began to be concerned that the constant sliding in and out of the paper envelopes might be causing some coin surface high-spot wear (paper is slightly abrasive) -- no evidence to support this -- just a nagging concern. 2. I didn't like cramming all of the coin background information into the small writing surface area afforded by the envelopes (and not having the provenance information readily available with the coin). 3. I wanted to see more of my coins rather than having them hidden within the envelopes. (although at home I do display them in those elegant velvet lined Abafil tray cases). So to my new system depicted in the photo: I use a zipper secured heavy duty (imitation) leather three ring binder (a section of which is illustrated) with regular plastic coin pages. An envelope is made up for each coin which has the basic RIC information written on the envelope face. All of the provenance information (previous owner tags, museum labels, etc.) is kept in the envelope. The coin is inserted into a cut-in-two regular PVC free flip -- the resultant "single flip" allows clear viewing of the coin and cuts down on the thickness of the full binder yet still provides snug and secure coin storage (especially during transport). The "assemply" is inserted into a pocket of the plastic coin page in the binder with the envelope (containing the provenance information) underneath and the coin in the flip half on top. I like the idea of the coin surface contact with the smooth plastic flip. I have included a depiction of a coin removed from its envelope/flip in order to illustrate the system. James 
Nice system, wish I was so organized...mine are mostly shoved into drawers or in a box in a safe. I also like the flips because you can see the coin better than in the paper. There is nothing on them that obscures the coin at all because I keep my coin info in digital format...those I have cataloged...others I have photos but havent cataloged yet but plan to. Its hard to keep up.