Bilious Bob and Homer, let's focus on the question, and not just dig at one another. There is enough vitriol in the media, we should focus our common concern for doubleguns here.
The question is which is likely a better investment, the Holland & Holland Royal or the Purdey?
The answer is neither and it depends. It depends mostly on what you pay for the gun and how long you will hold it. In the short term most, if not all profit is made on the purchase price and not on the sale.
The comment that high end guns can be hard to move is true. While some of us are positioned with friends and acquaintances, who can pay a fair price, many are not and trying to sell a high end gun at a good price can be a long and painful process. I do not believe that high grade guns are a good investment vehicle because they are not very liquid.
That said if one is to bet on the future market in terms of which gun is likely to go up in value more than the other between the two I think it is a draw. If I were looking to invest in a high end used gun as an investment I would buy a McKay Brown 20.
That said I have not the money and I am not in your league