I would go for 70 Watt SON floods, at 6 metre high these will give you an equivalent to street lighting levels with the right spacing. That should be adequate for the given task. If you choose a decent branded lamp , it should last at least 2-3 years. As most SON floods in the 70 watt range have symmetrical reflectors, they could be mounted central in the eaves and give an even light down the barn. A decent SON lamp can last up to 28,000 hours, compared to Metal Halide at 10,000 hours and T8 fluorescent at 10,000 hours.
If you need a more instant start up, I would go with T5 non corrosives with a good tube, the GE long life ones quote 30,000 hours. This will save on the cherry picker hire and help with running costs.