For argument's sake, we'll assume our Cavalier is Order of the Tome, though it makes little difference at first-level.
Things they have in common:
Everything not class-dependent: points to buy ability scores, racial abilities, traits, etc.
Same hit die
Same BAB
Same save progressions
Same weapon proficiencies
Same shield proficiencies
Same starting wealth
Same number of skill points per level
Most class skills: Climb, Craft, Diplomacy, Handle Animal, Intimidate, Linguistics, Profession, Ride, Sense Motive, Swim
Same general role in party
Advantages the Cavalier has:
Heavy armor proficiency
Less MADness due to less Dex dependence (and theoretically Int dependence if the Tactician plans on getting to level 15)
Challenge 1/day provided she follows her order's edicts
Mount
Bonus to Linguistics checks (or some other skill depending on order)
The actual Tactician class feature (bonus teamwork feat and ability to grant it to allies)
Class skills: Bluff, Knowledge (arcana, religion)
Advantages the Fighter has:
Class skills: Knowledge (dungeoneering, engineering, geography, nobility), Survival
Guess your Tactician had better hope that Knowledge (Geography) is getting rolled constantly. On the bright side, by eleventh level your Tactician will be able to spend an entire turn to Aid Another two allies as long as all three of you are adjacent to the same monster, so I'm sure the investment will pay off in the long term.
