I have no experience with Platinum Rolex models but I did own a Yellow Gold Yacht-Master for about 5 years and currently a White Gold Pepsi.
I have never had any problems with the 18K offerings apart from micro swirls which can be easily refinished. If you are already considerate to your watches I don't think you will have any issues.
As far as which watch is best for you Martyn it all depends on what other watches you have in your collection and which one appeals to you the most?
Is there a reason why you have the smaller size Day-Date as an option?
If it's down to price compared to the larger versions then this might not be the best option.