Your minor focus eats up mana more than your major focus. If you want to cautiously advance with your defense form and switch to your utility form for easier fights, go for defense major. If you want to advance as your utility form, go utility major.

Greater enlivens will affect your choise as well. As utility major, you can effectively negate the gimp factor caused by higher form mana cost with resolve of the mandrill (with resolve enlivened, your minor focus defense form can go from 50 mana/tick to 25 mana/tick or so). Utility will offer you that and other perks, such as anti-sleep, sneak, fast movement and reduced maledict stat penalties. Defense will offer you things like improved regeneration, improved dodge and improved dam redux.