Now that Nikon D 750 is back on the market, I am going to buy another one. They are selling the 24-120 F4 at $600 discount if purchased along with the D 750. I already have the 28-70 F2.8 and 70-200 F2.8 For a third lens, is $600 savings worth it despite being only F4 - or should I pass on the...