For the VLAD compact crossbow, the 30lb front end is the easiest to cycle. Suitable for low velocity indoor shooting (always use a proper target with a backstop to prevent over-penetration). 
The 60lb front end is the VLAD's standard accessory, fast to cycle and providing a decent amount of velocity (165fps).
The VLAD's fastest front end, shooting the bolt 210fps. The 90lb limbset requires a bit more effort to cock; but is still manageable for almost all operators. This accessory makes the VLAD a "nail-driver" and a formidable compact crossbow.