The other thing you have to remember is the big whites do not depreciate, and can be an investment, third party lenses are generally cheap to buy on the second hand market. Another reason to look at the big white's.
This is a good point & it has been my own experience, I owned both the version 1 300f/2.8 & 400f/2.8 Lenses, sold them for exactly what I had paid for them when I purchased the Version 2 Lenses, I could have sold them at a profit but I sold to friends so I was interested only in recouping what I'de spent.
Actually I disagree that. You have better think about that from finance point of view. Money actually loses its current value in the future because of inflation. For example, you buy 300mm L f/2.8 for $7000. You may sell it for 6500 ten years later. However, you will lose 5% inflation of 7000 yearly. That means the future of your $7000 is about 11500. You actually will lose not 500, but 5000. The more you invest in you lenses the more you will lose in the future.
Don't buy the big white because you think it will keep its value. You will lose more money on the investment. You buy the big white because you think it worth for the money. If I was a PRO, I would buy the best gear I can afford and bring return. Otherwise, I would not.