Magical Secrets is better, the newer you are to 5E. If you've only discovered Sharpshooter + Swift Quiver and Aura of Vitality, then those look like the no-brainer choices for Magical Secrets, and bards look overpowered. But then you realize that Wall of Force exists, and Conjure Animals, and Death Ward, and Web, and Spiritual Guardians, and Find Steed, and Bless, and... pretty soon you realize that all the spell lists have a lot of pure awesome built into them, and bards just get to tinker around the margins a bit. Bards start looking a lot more balanced at that point.
Since I refuse to play clerics for RP reasons, my Magical Secrets often go towards duplicating or excelling clerical powers, and Aura of Vitality is therefore my go-to pick. Beyond that it's a tossup full of agonizing tradeoffs: Counterspell, Spike Growth, Conjure Animals, Bless, Death Ward, Fireball, Evard's Black Tentacles, Web, Circle of Power are all attractive in different ways. Of course you always want to save one slot for Wish, for obvious reasons.