I think spare battery is better:

Pro:
Backup/replacement option (in case 1 fails).
Less bulk (compared to battery packsor the 2-in-1 battery replacement).
Charging is more convenient. After fully charge 1 battery, keep it as spare and plug in the other battery and charge and use it. Allows flexibility to charge when convenient.
I believe cost should be cheaper (provided you don't get a battery charger device).

Cons:
Inconvenient to charge and install (need reboot).
I don't think losing it could be a big minus point.