Cost: Nickel metal hydride batteries are, today, the less-expensive technology. As production of lithium-ion cells ramps up, though, economies of scale come into play and the cost of Li-ion cells should drop. When more vehicles require more batteries, each individual battery becomes less expensive to manufacture.
Weight: NiMH batteries are larger and heavier than Li-ion batteries. Weight matters in hybrid cars. Lighter battery packs with higher energy density make it easier to get the car going.
Power: Li-ion and NiMH batteries can actually hold a similar amount of power, but the lithium-ion cells can be charged and discharged more rapidly. Li-ion also doesn't have as much of a "memory effect," which occurs when a battery is recharged before it is fully empty. This can diminish a battery's capacity. Lithium-ion batteries are less affected by memory effect than NiMH batteries.
Durability: While both types of batteries are durable and both have been in use for years in various applications, this is the one area where NiMH has an advantage. Some Li-ion batteries don't last as long in extreme temperatures, particularly in very hot climates.