First of all, almost all Beneteau models are offered with lead keel options. If I remember correctly you are looking to buy a new boat so you can oder one the way you like. The last time I looked the lead keel options were pretty reasonable.
But back to your basic question,like most things in sailing this question is one of compromises. All keel types have some compromises associated with it. In the case of an iron keel there is a little more maintenance and the need to do the maintenance promptly. The problem of maintaining faring material is not unique to an iron keel. Virtually all external ballast keels (lead or iron) and many encapulated keels come from the manufacterer with a fairing material. These materials vary from manufacturer to manufacturer and can be anything from thickened polyester (body putty) to epoxy with glass fill and cloth. The quality of the fairing materials andthe care with which they are applied greatly affects the lenght of time between major refairing. Epoxy with a layer of cloth and minimal fillers can last almost indefinitely. Epoxy with some fillers can last 10 to 12 years between strip and refairing, and polyester fairing materials even with a barrier coat seems to last about half of that. Again this is seems to be true of fairing materials used on all of the keel construction types.
The thing about iron keels is that upon haul out you need to fairly quickly inspect the keel for dings in the fairing material. If there is bare metal or loose fairing material, it needs to be ground off and refaired because the loose area will spread pretty quickly as rust forms below the fairing material and prys loose a larger fairing area.
At some point, all of the fairing material needs to be stripped and redone on either lead or iron or encapsulated ballast. Iron usually happens sooner and needs more extensive fairing work as the surface of the cast iron is rarely as fair as the surface of a cast lead keel. If I had a boat with an Iron keel I would probably include a later of fiberglass cloth in the fairing process to increase the strength of the fairing skin.
But before this chases you off looking for an encapsulated keel many of these have external fairing materials as well. This is another one of those ''no one universally right answers'' questions. In other words an argument can be made for either type of keel. (For the record, I personally strongly prefer a bolt on keel rather than an encapsulated keel.) Here''s the way I see it.
Bolt-on keels tend to offer more performance since the ballast must be cast and without the keel stub skin thickness tend to be lower relative to the center of bouyancy. They also have significantly less wetted surface and frontal area making them theoretically faster on all points of sail. They are simple to repair and generally can be repaired satisfactorily no matter how bad the mistake.
On the down side they are more expensive to build; requiring precision casting, bolt hole drilling and a lot more hand fairing. They are higher maintenance requiring fairing every 10 years or so and new keel bolts at some point in the boat''s life.
Encapsulated keels are less expensive to build. There''s less labor and less precision required. Boat builders will often use less expensive forms of ballasting with encapsulated keels, such as iron or lead scrap cast in concrete, resin or other binder further reducing costs. If they are not damaged in a grounding, encapsulated keels are less expensive to maintain.
On the minor down side they are less efficient from as sailing standpoint. Their real downside is the difficulty in doing a proper repair. Typically, in a hard grounding a number of things happen on an encapsulated keel. Typically the skin of the keel encapsulation gets ruptured and separates from the ballast. This allows water into the small cavities between the keel and the ballast and once wet it can mean the ''beginning of the end'' for the boat as this permanently wet fiberglass blisters itself from the interior and the wet areas spread around the ballast. This is especially a problem on a boat that is hauled out for cold winters where freeze/ thaw cycles can really pry the skin loose from the ballast. The problem gets worse when the ballast contains ferrous materials. Here the ballast begins to rust and can reduce the ballast into a loose mass of matrix and rusting iron.
Beyond that, in a grounding the ballast is often forced upward as well. In an encapsulated keel the membrane of the hull is at the outside of the ballast keel and the membrane above the ballast is often quite thin. In a bad grounding the ballast keel is often is pushed through this membrane causing serious and difficult to repair damage and leaks.
We grounded a boat with an encapsulated keel that we never could permanently fix for as long as we owned the boat. The problem would get worse with every year, spreading from a small dimple on the leading edge of the keel to an area that was much of the bottom and sides of the keel.
Lastly, it is very hard to lay-up the glass in the keel cavity. As a result the glass work in this vulnerable area of the boat is often inferior to the glass work elsewhere on the boat.
So as I said, there is no single right answer here. You just choose your poison. Mine is a cast lead bolt on keel but that does not make it the only right choice.