I'm new to plant growing but I understand a decent amount on lighting due to my old hobby, reef keeping (I use to grow coral in my living room). Here is a simple breakdown of LED and metal halide.
LED:
Pro: better at producing specific wave length, strong par/watt ratio, less heat (but still produces a decent amount of heat), bulbs are more durable (harder to break and last longer), directional light so it shines on whatever area you want it to
Cons: initial cost although prices are significantly less than what they were years ago, because they are so directional, you will need more to cover a larger area, not as powerful as MH,
MH:
Pro: strong light that can pack lots of punch, can be purchase at big box stores, produces heat so it can provide frost protection, not directional so 1 bulb can cover a lot of area (I plan to use 1 250 or 400 watt bulb to light my whole garage up. This will keep it frost free and give adequate lighting for all 30 or so plants.),
Cons: you need to replace the bulb every few years because the bulb wave length will change as the bulb burns and gets older/used, easier to break as they are made of glass, not as precise in color spectrum (but I think it's healthy for plants to have a arrays of colors vs a few specific light colors),
I personally think it's a toss up on cost. LED cost more to buy initially vs MH. However, 1 MH can cover a lot of area. I had used 2 LED grow fixtures to cover my plants last winter. This year, the plants are bigger than last year. this same area would easily be covered by a single MH. The cost to purchase 2 units vs 1 is a consideration. Also, the cost of running LED bank vs 1 MH cancels that price point out too. The MH bulbs in the aquarium hobby are good from 8mo to 14 mo running around 8 hours a day. After that, they don't produce the wave lengths and algae will start growing.
From my aquarium reefing days, I observe way better growth with MH than LED. The MH just had more punch to the light. This is why I will be digging out my old MH system and will be lighting my plants in the garage with a single bulb.
ThangBom