The numbers represent the power. A Two is a better gun than you'll ever see in a Piece, but not excessively powerful. Up to Ten you get special traits, five being remarkable and ten being to the extreme. Once you hit the Faces, you get actual abilities, the capacity to do something that just isn't possible, like shooting more than six shots from a Diamonds six-shooter or having control over the spread of a Spades shotgun.
The suits do matter, and they work like in poker. Dimaonds, Spades, Hearts, Aces. Still, like in poker, having good cards is one thing but being able to play them is crucial. If the Gunslinger is better, they can outdo a better Card.
Diamonds are the weakest. Speed is good, but being lucky is better. A lucky quickdraw beats a fast quickdraw. A lucky shot can shoot down incoming rounds. Lucky can do things speed can't.
Clubs is good, but Hearts beats it. Lucky only makes unlikely killshots more likely, accuracy makes them certain. Being lucky is like being good sometimes, being good is being good all the time.
Hearts loses out to Spades. Spades, and sheer power, trumps. You can kill a Spades-holder first, and you had better pray you do. Once they fire, you are dead. You can't trickshot a Spades bullet out of the air, it has too much force for that. You can't outrun them, or dodge them, although if you're fast you can be out of the way before they fire. You can shoot a Spades-holder all you like with Hearts, but unless you kill him before he fires, you're dead. Brute force trumps.
Naturally, though, the nature of these sutes is such that the weaker a suit is, the less likely the user will rely just on it. Spades users become overconfident more easily than hearts, more easily than clubs, more easily than diamonds. Being a bad gunslinger is not something you can compenstae for all the way just by having a more powerful Card.