This is what the BMC say:
"Until recently there was no standard for belay sets or components, so purpose-made belays from bolt manufacturers were made from EN959 bolts with additional components obtained from a variety of sources.
This changed with the recent update to the EN959 standard, which now includes a standard for those belays which include two bolts linked together, usually by a chain.
Whether bought complete or assembled using components yourself, there are two important rules for belays.
● Always use components made from the same material, otherwise the belay will be subject to a dangerous form of decay called galvanic corrosion. Typically, this means components and bolts which are all made of AISI 316 grade stainless steel.
● All components should be rated as load bearing. A minimum breaking load of 25kN is given in EN959 as a requirement for all components of a belay. This means that any additional components, which may be subjected to wear, are at least as strong as the
anchor bolts in their strongest orientation.
There are a number of different formats and options for lower-offs, and which is best depends on a number of factors.
The simplest format is a pair of belay bolts. These are either resin bolts with a larger eye or expansion bolts fitted with a special belay hanger. Each has enough room to thread a bight of rope through whilst having a karabiner also attached to them and provides a rounded radius which doesn’t damage the rope as the climber is lowered.
These have a low visual impact, which is useful for venues where bolting is barely tolerated by the landowner, and they also give the most placement versatility which is important when the rock quality is poor.
On the more popular routes and crags, wear of the lower-off bolts can become a major problem, particularly where bottom-roping is common. In these cases, it is important to have some form of replaceable and hard-wearing component to lower-off from, which when worn can be replaced leaving the bolts themselves intact. Historically climbers have left old karabiners on the bolts, but these rapidly seize up and decay because of galvanic corrosion.
One option, which can also be retro-fitted to existing belay bolts, is to attach a belay ring to each belay anchor using a stainless steel maillon. The rings last for a long time as any wear is evenly distributed as they rotate in use. Thread-lock glue on the maillon prevents casual theft, but still allows removal with a spanner when the ring eventually needs replacing.
Maillons used alone are cheaper, but wear faster and have a tendency to be undone by movement of the rope.
Overall, the ring and maillon arrangement has much to recommend it, but there are two downsides to consider. The most important one is that the rings must be threaded, which requires the climber to untie from the rope. This creates a safety hazard both for novices untrained in how to do this safely, and for experienced climbers who may make an error.
Secondly, the rope going through two points tends to cause twisting of the rope, particularly if the belay points are level with each other.
There are other options available which can help solve these problems. Linking the bolts with a chain allows a single point to be used to lower from which prevents twisting of the rope. Curled metal shapes called rams-horns allow lowering without having to re-thread and untie the rope. Stainless steel karabiners give a good balance of security and ease of use.
Before considering any of these options, bear in mind that they will have some downsides of their own, and often an increased expense."
Untying the rope is not an issue that we have to worry about, but we are concerned with twisting, expense and aesthetics (my highlights). It would be interesting to know if any research has been done into which direction is easier to pull down a rope from the vertical bolt above, horizontal bolt below orientation.