I have a Pietta version of both the 1858 Remington and the 1860 Colt so I can give you some of my percieved pluses and minuses of each...
First my take on the Remmie...
It's more like a "modern" design with it's solid frame and grip assembly all in one piece. The cylinder is larger and probably stronger. Cylinders are super easy to change. The"drop-in" conversion cylinders are really fast to change with a little practice. Grip panels are more standard and interchangeable without fitting and sanding. Standard sights are usually high enough so if the gun shoots low a little bit off the front sight will put you right onto point of aim. Remmies are easy to cap and tend to be pretty reliable. They seldom "eat" caps and jam-up. The skinny hammer nose and narrow slot helps prevent this.
The downside of the Remmie is cleaning and ergonomics. The grip is too close to the baskside of the trigger guard. Uberti versions are really bad in this respect. Hammer effort is high and the trigger, while nice and wide, feels too short. The whole revolver exudes a sort of Victorian "clunkyness"; charming but sort of awkward. They are harder to clean and less tolerant of powder fouling.
Colts are beautiful. They point very naturally and the large "Army" size grip feels perfect in my hand. It is easy to cock the hammer but the trigger is really skinny and strangely offset in the triggerguard. Colts don't have a unitized frame. They are made up of a bunch of screwed together hand fitted parts. No two are exactly alike so grips have to be custom fitted to each gun. The arbor wedge holds the whole gun together. The upside to this is they are really easy to clean the powder fouling out of. You can pull the barrel and cylinder off of the frame/action and dump the cruddy bits into hot water/solvent/cleaner. Colts are really designed to cope with the filthy realities of black powder shooting. Their most aggravatiing aspect is the wide hammer slot and the frequency of cap jams. Cap fragments can fall right into the action and totally jam things up.
I think 19th century caps were a lot thicker than the ones we shoot today. Some people are really put off by the barrel wedge issue. It has not been any problem with my Pietta. The arbor "bottoms-out" in the barrel recess so the wedge doesn't have to be adjusted to prevent binding. I just tap it in far enough to keep it from shaking loose. It's not really tricky at all. Colt front sight are always too short. Navy models are really bad in this respect. The 1860 Army has a brass blade instead of a little "acorn". I soldered a little piece of flattened brass channel stock onto mine to adjust the point of aim. Opening up the sight notch on the hammer nose also helps and can lower the impact point a little.
I guess in summary I'd have to say the great looks and superior handling qualities of the Colt just about makes up for it's shortcommings. The Colt will need more fiddling and finessing to get running smoothly. You have to try different brand caps etc. until you find what works.
The Remmies usually work fine right out of the box. I have to agree with the "experts" who recommend it over the Colt for a first time cap and ball shooter. It is a easier transition for a cartridge shooter to make.
Hope this helps your decision. Cap and ball revolvers are not too expensive so maybe you should do like I did and get one of each !