It's hard to weigh the merits of buying a horse when I have no idea how they compare to the one I already have. Am I missing something?
There are only three horses worth investing in.
- Kentucky Saddler (avg speed, avg damage resistance)
- Hungarian Half-Bred (slower speed, higher damage resistance)
- American Standardbred (faster speed, lower damage resistance)
The speed and damage resistance noted are in relation to each other, when comparing these horse to any other in the game all three have higher speed and damage resistance.
Screenshots of the different horses allow you to determine which one you currently have because all the horses of a certain breed have the same color patterns, or you can compare what's in the store to what you have.
If your curious about the different horse breeds, http://reddead.wikia.com/wiki/Horses offers all the stats for each breed of horse. Screenshots of the different horses allow you to determine which one you currently have and if you really want to pony up the money for a better one. It also has locations in the wild to show you where to catch the horse, if you have not yet unlocked the deed for that particular horse.
The site also shows horses that can not be bought or caught such as the Dark horse and the pre-order War horse.
I believe that it is the case that all the horses of a certain breed have the same color patterns. So if you're looking for a particular horse, you can go scouting for one, and when you see the right color pattern, you should be good to go. For instance, I believe that the all white horse is the Hungarian Half-Bred.
Conversely, if you're curious about the horse you have, you can go to a store, and compare the horses they have available with yours...if you read the descriptions you can find the one that matches yours. Roughly speaking, the price will reflect the overall value. (This won't take minor differences into account...as Jon mentions, there are three horses that sell for $750, and they have slightly different statistics from each other...but all are better than their $250 or $500 counterparts).