Sailors may love RO-ROs but soliders love HSVs...
It is a shame that the end-user isn't the one making the decisions. I am about to slightly derail the conversation at this point, just regarding sailors and soldiers and their "grunt on the ground" wishes.
This is purely based on personal experience and also on extensive contact with current "end-users", I can categorically say the following...
Soldiers - A list of things that the grunts love aboard ship:
1) The excellent food. The cooks aboard all Navy ships work their butts off to do the best they can with what they have. Soldiers always appreciate the high quality food, particularly because once they get off at the other end, they may not eat as nicely.
2) Running water, regardless of how little they are allowed (if on the way back fresh from deployment)
...and that about covers it.
A list of things they hate:
1) Constant damage control drills.
2) The extremely cramped conditions.
3) The lack of space to conduct personal fitness.
4) The limited freedom of where one can go.
5) Having menial tasks to 'keep you busy' while they lose their 'work skills'.
6) Boredom.
7) In some cases, seasickness. (Remember they aren't sailors by trade)
...amongst many, many, more. Too many to list, some which I can't talk about, (some which would surprise the folks at home in the armchair, too).
Any embarked troops mess aboard a ship is cramped, uncomfortable, and is merely a place for cramming the maximum amount of live cargo. It matters not how big the ship is for the most part, because the simple rule of thumb is 'X number of troops per Y-cubed amount of space'. Quite simply, the rule of thumb is "the bigger the space, the more grunts will be packed into it", regardless. At night, it can be oppressive given the sheer number of troops, the noise, the odd sick or noisy person...
If you ask a soldier how many days he is willing to spend aboard, his most likely answer would be either "As few as possible", or "How many of those will be in a friendly port?"
Sailors:
They like things that go quick - not just for the "big man" factor, they love it because of the ability to put more nautical miles between them and anything you care to mention.
They love shiny new things, and don't like to be accused of driving a floating bus (which is how a RO/RO is viewed).
They have a friendly rivalry with birdies; a lack of helo operations won't bother the crew of a ship too much (less stuff to worry about).
They love to get rid of embarked troops as soon as possible, the shorter the time the extra bodies are aboard, the quicker they get their space back.
Ask any 'end-user'... if you gave them a choice, they'd usually pick the 'Cat. The Army as a whole would love 'Cats for the speed they can put down, and the speed they can rotate troops to and from the zone. The Navy guys, although they aren't hugely picky, would prefer to be posted to a 'Cat than a RO/RO.
Then there is the whole "cool factor" of what we can do publicity-wise with a sleek, fast and seemingly cutting edge vessel. Point at a RO/Ro and tell someone how well engineered it is, and most people will look at you as if you are nuts. However, a glance at a 'Cat, and most people at least think "Hey! Cool!"