Behringer pedals low prices are tempting but they have not been proven to last, so the best to do is to have a wait and see attitude.
Boss pedals are still the best, they can last quite a bit and have a better resale value.
The GT2 is expensive is because it has more functions, its a amp modeler and a DI, and hence the higher price, it is also a sturdy pedal which also has a better resale value. You can get them second hand easily also, every now and then there will be people sellin them at luther ads.
The GDI21 is 1/3 the price of the GT2 because they simply copied the thing, there is no cost involved in developing the product.