hoo boy. Okay, there are 2 things in this game that matter above all when it comes to cards and team building: leader skills and passives. Leader skills come in tiers, but the good ones typically give +3 ki and 50/70/120/150% to HP/Attack/DEF to all cards of one type or category. Generally, whatever your most powerful leader is, that's what you wanna build your team around, because the stat boost is the most important determiner of team strength. Card color is a secondary concern that should only come into play once you have leaders of equal strength for multiple types, like say a 120% super STR leader (SSJ4 Goku) and a 120% Extreme PHY leader (Cooler). The other major factor in team building is link skills, but again, this is secondary to making sure that every unit on your team benefits from your chosen Leader Skill. By and large, if you think 2 characters should link well together, because they're both super saiyans, or because rose and zamasu are basically the same person, or because cooler and frieza are bros, they generally do. For example, many SSJ cards have Prepared for Battle, which gives +2 ki. Assuming you start with 6ki from leader skills (yours and friend's), that means you only need to get 4ki to Super when that link is active.
Passives are the biggest determiner of card strength, all else being equal (assuming cards are awakened and max level, as that's pretty much instantly doable once you've been playing for a little while). Passives come in many shapes and sizes but the rule of thumb is: % attack boosts are the best, the higher the better. The good cards typically have 80/100/120% attack boosts on their passive. Flat boosts, like, say, +7000 attack, are a lot less powerful because they don't scale as well with leader skills (and potential system buffs, but this doesn't come into play until rank 50).