matador,
I think you have asked two very seperate questions here, and the answer to one is irrelevant to the other.
Firstly you ask about rotor designs and the resultant downwash issues. To oversimplify the issue, higher disk loading, higher downwash (or more intense) and I think this is the thrust of what you are really trying to ask. But as Nick and others have said, other factors are at play, and all design is aimed at finding the best compromise to achieve the desired mission profiles, thus the question you actually asked is extremely complex.
As to fire fighting and downwash, I believe it is largely but not totally, a pilot issue. Firstly, IMHO, to oversimplify my first oversimplification about disk loading, the heavier the machine, the higher the disk loading and thus more intense the downwash (exceptions here are plentiful, like Kmax, and machines like Bell 205/212 has 17% slower downwash than BK117 even though it is heavier). BUT, the heavier the aircraft, the less they need to be hovering over a fire, thus if the pilot is on the ball, downwash over and active fire is not really an issue.
Where downwash does become an issue over the fire is where pilots do not understand optimal drop profiles for the machines they are flying. Because most of us learnt to fight fires on smaller machines, it is hard to break the small machine required habit of coming to a hover or very low speeds before you drop when you move up to the medium and heavy machines. Hover drops in mediums/heavies are GENERALLY a waste of water unless doing mop up (in which case it is likely to be a waste of money!).
Each peice of dropping equipment has it's own profiles dependant upon foam densities, foliage penetration requirements, and fuel loadings. Thus a bambi bucket has different profiles than a belly tank, and Isolair will have different profiles than Simplex. It may suprise some of the fire pilots out there, but medium bambi bucket machines (say 1000L to 3000L ) and above generally have an optimal drop profile at around about 30kts/30ft! (That's 30 ft bucket hieght). Even in dense foliage the speed recommendation is not below 15kts, and in the grasslands, it may be as high as 45kts. But time and time again, you see mediums come to the hover or less than 15 kts at a hundred plus feet to drop. At the recommended speeds, downwash is not a significant issue on the fire, the issue only really arises when you have the incorrect techniques and excessively low speed drops from larger aircraft. So you can see that for dropping on active fires, downwash is a pilot issue rather than an aircraft design issue.
Mop up, winching, and water pick up are all hover required (or low speed required) fire tasks, and the design of the aircraft is important here in terms of downwash. The higher the downwash intensity, the more risky each of these manoeuvres becomes.
High downwash during the mop up has the potential to strip fire weakened branches down on to personnel, generate ash to destroy viz, kick up coals and rekindle the fire. Thus low downwash machines are more suitable for this phase of the operation (B206 series and Kmax are ideal - Kamov is silly).
Winching ops is similar: the more intense the downwash, the more risk to people on the ground with debris, srtipping branches, spin, and viz issues. I have also witnessed winch aircraft start a perfect ring of fire around there winch point! But winch aircraft have a compromise here, and thus choice of the machine should be balanced with other considerations such as twin, or single, winch type fitted, crewman station, number of ground crew capable of being lifted, etc. These other considerations rule out machines like Kmax, and reduce the economical effectiveness of machines like the Bell 206 series. I would suggest that downwash becomes the over riding issue when we start to talk Kamov, Chinook, etc.
As for the water, all the machines have to get water, so downwash is just one of those things. Some times in Australia it can be an issue due to the dust around dams, etc, but even this can be overcome with a quick drop around the dam at kick off.