I have both games, and they are my systems of choices. They are completely different games with almost nothing in common (only that both were derived from d20, and a few common things like having levels, BAB, or the six ability scores).
1) Castles & Crusades
Looks like AD&D 1e/2e that would have been streamlined using the d20 philosophy. The usual classes (Fighter, Ranger, Paladin, Monk, Druid, etc.) but simplified, without skills and feats. Saving throws and task resolution (skills) are resolved using an ability checks system called the "SIEGE ENGINE". Magic operate as in d20/AD&D (clerics, wizards, illusionists who prepare their spells before casting). Overall the game is very rule-lite and fast to run, but lacks customization options. For example, multiclassing is not explained, and in any case would be as in AD&D, not d20.
Otherwise monsters are very simple and easy to run, not heavy stat-blocks bloated with info as in d20. Also, this game was designed with the idea that DMs should houserule it easily. There is two main rulebooks at 20$ each.
2) True20
Three classes (called roles): Warrior, Expert, Adept. As you get one feat per level, and multiclassing is without restrictions, it's easy to make very different characters almost exactly as you want them to be. BAB and saving throws are as in d20, but the major difference is with the wounding system. Characters don't have hit-points, and you don't roll weapons' damage dice. Instead, weapons and spells add a bonus to the attack roll, and then the target must make a Toughness save. If this save is failed, they get a "wounded condition" that induces a penalty to his next rolls. The more you ae hit, the harder it becomes to fight. I must say that many people find this way of handling damage a bit complicated (more than counting hit-points). Other things: ability scores don't range from 3 to 18, but only use the ability modifiers (-5 to +5); there is no XP (leveling is left at DM's whim); magic is very different, not using memorized spells and spell slots, but powers that are flexible, and can be used as much as you want so long you don't get fatigued using them. Also, there is no disctinction between arcane and divine magic.
Monsters resemble their d20 counterparts, with slighty lighter stat-blocks. There is two main rulebooks at around 30$, but also available in PDF. There is a few optional supplements.