I have little direct experience but I have been doing a bit of research while I wait for my FAC to come through. Hopefully wiser people will correct any mistakes...
Couple of web sites that you may find useful
http://www.muzzleloading.co.uk/
http://www.charcoal-burner.com/
http://mlagb.com/
Calibre diesn't seem to be a particluar issue for most competitions, the key factors are single shot, revolver, and spirit of the original. Few people can afford to own, let alone shoot, a genuine original, so most use a reproduction from the likes of Euroarms, Pedersoli, etc.
The MLAGB run national competitions (using very similar rules to Internationals) where there are usually classes:
Kuchenreuters - single shot, spirit of the original, 25m
Mariette - revolver, spirit of the original, 25m
Club competitions may include similar disciplines and others such as Bow Street Runner, Cavalry Officer. Generally the rules are more relaxed and "spirit of the original" is not usually an issue. This "Spirit of the original" business is a bit open to interpretation but usually means that a modern replica must conform to the operating & design criteria of the original - in particular that the sights must be fixed on the replica if they were fixed on the original. Typical pistols that can fit this criteria are Rogers & Spencer and Remington NMA. The most common calibre used is probably .44. Be aware that e.g. the Euroarms Rogers & Spencer is available in a number of versions, some with traditional sights, others with modern adjustable sights or non-traditional foresights. Colt pistols are less common in competition as the lack of top strap tends to make them less accurate as they "stretch" and "wear" over time. However they are often considered the most "pointable" of all American civil war type pistols.
The most common revolver for competion where "spirit of the orginal" is not an issue is the Ruger Old Army, this is classed as a .45 for practical purposes, including FAC, even though it might be more properly be considered a .44. This was designed as a new pistol in 1970's by Ruger using all the advantages of modern materials and machining. It is generally considered to be absolutely bulletproof (pun intended!) and will last a lifetime. No longer available new, but there are lots available S/H in good condition.
Choice may depend on what sort of competions your local club run, and whether you want to progress to National stuff or just want to have fun with some marvellous pistols.
cheers
Nigel