First off, you need to decide if your reel seat will be up-locking or down-locking. Working with a rod maker, decide which seat you prefer or which is best for the rod you're having made. Each style has its pros & cons, but the down-locking reel seat is the traditional style. The popularity of the up-locking seat (which has an inlet in the back of the grip to accept the reel foot) is a fairly modern fashion. For many decades the down-locking seat was what was seen on most fly rods.
The next thing to consider is the locking mechanism for the seat. Here you have two choices: slide band (cap & ring), or screw locking. Some folks just feel safer using a screw lock seat, but as long as it's well designed, a good cap & ring seat will work quite fine to hold the reel to the rod. There are many antique heavy bass & salmon rods that employ a slide band seat. Some only use two rings to secure the reel to the seat. If the seat is made well with quality in mind, there's no reason not to consider a cap & ring seat.
The insert, or spacer (main body), of the reel seat is another thing to think about. Here you have tons of choices & there seems to be a never ending supply of different materials. The traditional choices are between wood & metal. In the late 1800's & early 1900's it wasn't uncommon to see an all metal reel seat. You can still find these today, especially with an all aluminum spacer. They are particularly popular on spey rods or for any rod that could be exposed to salt water. The wood spacer, however, has been the standard choice for reel seats for many years & remains quite popular today. A wooden reel seat spacer can add a “high class” look to your fly rod. Consider it the equivalent to leather interior in a luxury sedan. The choices of woods used in reel seats runs the gamut of standard American hardwoods, to exotic foreign woods. Walnut & other woods like cedar have been the traditional woods used through the ages & are still good choices today, but you can also have woods like bubinga, teak, bocote, cocobolo, bird's eye maple, etc. Today reel seat spacers are also made from modern materials, like the popular woven graphite material. These can come in a variety of colors & are very durable. However, with split-bamboo fly rods you should consider aspects of tradition & not opt for a modern material on a bamboo rod. Still, the choice is yours.
One of the debates when choosing a reel seat in regards to the metal used is the choice between aluminum & nickel-silver. Contemporary rod maker Jeff Wagner has this to say about the subject:
"Aluminum is a totally acceptable reel seat metal, and in fact carries some advantages. First, it is considerably lighter than nickel silver, yet when highly polished it takes an expert to tell them apart. It is also considerably less expensive. The use of nickel silver in screw lock seats is a recent phenomenon, and such esteemed makers as Payne and Leonard used aluminum in screw lock seats. It's hard to argue they did so to cut costs or corners."
I offer fly rods in different grades, or levels, of price. One of the reasons I can do this is by using reel seats with aluminum hardware for the less expensive rods. What the aluminum allows me to do is offer a rod to a broader range of anglers who want a custom rod, but are on a tight budget or to those that have a limited amount of money to spend on a rod & want to invest it into another rod feature. These folks get a rod that is similar to one sporting a nickel-silver reel seat but without compromising on functionality or detracting from the overall appearance of the rod. Not to mention on some rods (no matter what their grade), aluminum is the best choice. This is critical with very light weight rods.
As you can see, your choices abound when deciding which reel seat to have on your fly rod. I highly recommend discussing reel seats with your rod maker. Bounce ideas off of them. Remember, these are choices you don't get when you purchase a fly rod from a corporation - you get the reel seat they've decided on for you without your input. Most rod makers will be happy to share with you their thoughts about reel seats & help guide you in making your decision.