The starting point is to make sure that ribbon mics are the right thing for your purposes.
Ribbon mics are so called because they are based upon an extremely lightweight metal strip (or ‘ribbon’) suspended between two magnets. As the strip vibrates with air movement, a small electrical charge is generated producing a signal.
Ribbons move in three dimensions so are extremely realistic and musical. Most start to roll off quite dramatically at around 9KhZ, so ‘soften’ any instruments with a tendency to harshness or brashness (such as brass, many pianos and cymbals). Indeed, I’ve often heard ribbon mics described as sounding as if they already have eq and compression added.
Almost all ribbon mics have a figure of 8 pick up pattern (Beyer and AEA do make cardioid ribbons, though) so don’t offer much if any separation. As such, they have drawbacks for ensemble recording unless the players are heavily screened. They also need ‘air’ to breath; in other words, if you intend using as overheads on drums you’ll need a good ceiling height to be able to suspend well above the kit. In cramped spaces, the lack of directionality will usually result in a form of scrambled audio soup although you can use a ribbon for an ambient mic placed a few meters in front of the kit rather than above. Experiment and take care, though.
By and large, you’ll need a decent preamp, although this is nothing like so critical with modern ribbon mics as in days gone by, when ribbons had very low output and often a very low impedance.
In terms of choice, different ribbons have different pros and cons.
The larger the magnet, by and large the greater the sensitivity (i.e. the output level) and the frequency response. However, the lightweight metal ribbon is prone to damage from loud signals (kick drums, guitar cabs) or wind noise. Certain mics (such as Coles 4038s or RCA DXC44 and 77) should NEVER be used for these purposes. However, in recent years manufacturers such as Royer have produced mics with stiffer ribbons and heavier suspensions specifically tailored to micing up guitar cabs and dealing with higher level signals. The trade-off tends to be a less natural and sensitive response.
As with any mics, it’s wrong to compare one make with another just as it’s wrong to assume that the more expensive a mic mis, the better it is. All mics have their virtues, but one size will never fit all. A cheap ribbon mic might be perfect for a particular application but suck big time for another. Similarly, my favourite Coles 4038 is (to my ears) unrivaled on pianos or drum overheads (in pairs) but can’t compete with a Beyer M160 for spot micing strings in an ensemble setting.
So, as with all mics it’s down to the preferences and ears of every engineer to decide what works best for him or her. And such is the joy of recording – there are no hard and fast equipment rules apart from the old adage… if it sounds good, it is good.