Firstly, the GSM vs CDMA phone is a thing of the past. Apple first started to make "world" phones by incorporating both radios circa the iPhone 4, and it stopped being an issue in general once 4G LTE became the standard and achieved widespread penetration. When one says they're "on" Verizon now, it only means that they're getting service from their network of towers, not because their phones only speak CDMA.
The 2018 iPhone XS was the first to accommodate eSIMs, and the budget SE2 got it in 2020, so anything newer won't be an issue.
Like many things, how people approach these sitations is largely a matter of their tolerance for convenience vs cost.
The simplest would be to do nothing, rely on one's native home carrier, and pay through the nose for international roaming charges.
However, being on an MVNO, most of them do not offer roaming, never mind international roaming, and even then it's usually restricted to North America, though there are exceptions like GoogleFi, subject to
conditions.
Next easiest option would be to opt for an eSIM, which can be purchased online in advance, and can be activated once on the ground (though one should pay attention to their advice on when to activate). The main disadvantage to this option is that they are data-only plans, and don't have a local phone number and no SMS/MMS. But assuming one is not a luddite, or has particular reason to need a local phone number, voice calls and texts can be accomplished by any number of options, whether FaceTime/iMessage, or the popular IM apps. It also has the advantage of having both services present on the phone simultaneously, making it simple to switch between them.
Buying a physical SIM does give the option of a local phone number, and offers the greatest control over which local carrier is chosen, for reasons such as service quality and coverage. The EU does not allow roaming charges within its territory (sorry, UK), so a SIM purchased in Italy can be used with no problem or additional cost in France, albeit with an Itailian phone number. Not sure why anyone would try to open a can of worms by messing with home phone numbers and foreign carriers, so I wouldn't even consider such a possibility. iPhones use their SIMs only to provide service, and don't store data on them in the normal course of usage, so that's not an issue either.
Each country has its own rules with regard to SIM purchases, and may require additional hoops to comply, so bear that in mind. That's part of what may make pre-purchasing a SIM, or eSIM plan while at home preferable, though again, it might come at a higher cost for that convenience.
One thing I forgot to add -- starting with the iPhone 14, the models sold in the U.S. no longer have physical SIM slots; they are eSIM only. Models sold in the rest of the world can still use physical SIMs. As it was with nano-SIMs, Apple is trying to nudge the industry to adopt eSIMs, and eliminate physical cards altogether. The carriers wouldn't mind this either, since eSIMs provide an additional measure of control, but being incumbent members of a cartel, things move only as fast as they like, slowly.
Which option is best will depend on one's needs, and preferences, so there is no "best" except for what suits you most, based on the itinerary.
