A 270' yacht is a good size to support a helicopter, and even a "white" yacht style design should be able to aesthetically integrate a private (ie. not to commercial standard) helideck for a A109 Grand, but a Dauphin may be pushing it a bit (although expedition style yacht tend to be able to dedicate more space to such things as helidecks, so if it is that sort of design it may work)
The shipyard/designer will be building the yacht with the largest helicopter intended in mind - so it would be a good idea to find that out.
Both the helicopters that you have mentioned have pros and cons - as do all helicopters. Which one is right for the yacht is only part of the question, as you should think about all the other aspects of the helicopters use. So really the question is - which helicopter is the best for the owner, and will that integrate well with the yacht?