Most people really know very little about music and the craft of performing music. Even fewer have any knowledge of classical music. So, I thought I would write a little about the process of matching your musical needs and desires with the musicians who can fulfill the realities of your musical wishes. The factors you should have in mind are; budget, preferred musical styles, venue, number of guests, and certainly the quality you are seeking.
First, before you start shopping for a trio or quartet consider your budget, is it really more in line with a solo or duo or are you really looking at a quartet. These are approximate ranges (my fees are generally within these ranges) but solo music for a ceremony in the Charleston can run from the low end at about $200 to $350 or more on the higher end (especially for a solo harp). Duos run from About $400 on the lower end to $700. Trios and quartets will obviously be more. These are base fees that don't include extra travel costs that are sometimes charged for venues in the surrounding areas, and extra fees that are sometimes applied for learning non standard selections or for accompanying a singer. Fees are also often quoted based on your own unique situation, such as instruments, time of year, Friday or Sunday vs Saturday weddings.
Second, think of the styles that you want and the over all feel you want to convey. If you want something classical and elegant (think classical guitar!) don't hire someone who primarily sings and plays popular music. If you want mostly popular music with maybe Canon in D and Jesu Joy of Man's Desiring thrown in then look for someone who has the popular selections that you want on their list. Maybe even use the singer/guitar player from the band that you are hiring. If you want loud and festive, think brass! Some groups have the ability to do multiple things well and every group has its strength. Playing classical music well takes years of training and commitment. Playing popular music well entails a good sense of feel and style. Also keep in mind that most of the music that you will want will need to be "arranged" for the format of the group. For example the traditional Bridal Chorus by Wagner (Here Comes the Bride) is from an opera and was originally written for a massive orchestra. When I play it on the guitar, I reworked or "arranged" it to fit on the instrument so that you can hear the melody, harmony, and rhythms of the original. In the case of a pop song, the original usually has a singer along with some combination of guitars, keyboard, bass, and drums. All that needs to be reduced to a new format where the song will exist in a playable format for entirely different instruments. I must say that one strength with my groups in particular is that the presence of the guitar allows me to get some of the proper feel to a pop song while filling out the harmonies of classical selections.
Venue and number of guests go together. Solo guitar works wonderfully in smaller spaces with shorter guest lists. String quartets or even brass ensembles are great for larger spaces with a huge guest list. If you want really loud, go with bagpipes! I have played solo guitar in very large venues with huge guest lists, that's just the sound and feel that they were after and so I went through a main PA system. Generally speaking though, if you want a more intimate feel, go with a smaller group. With more guests and larger spaces, go with at least a duo or trio and perhaps if your budget allows, even a quartet. I did have one very interesting wedding where a couple who worked in the movie business wanted my trio with an added trumpet and french horn to play an arrangement of Star Wars and Superman. The groom even came out in a cape. (This really is a true story!).
Presumably, you will be only married once or if you've been married before, this is the last one. I think the most important thing to consider is how they sound! Listen for the colors of the instruments that you like. Those sounds will form the soundtrack for your wedding ceremony. I feel really privileged to be a part of such and incredible and intimate event. As partial as I am to the guitar, I'm a music lover and I'd encourage you to pick the sounds that make you happy! Wishing you all the best with your plans!