To be honest there is no real right or wrong answer to this question as what will suit me or other may not suit you. What I recommend is heaps of research and know the product, hire and test for yourself if possible and don't be scared to ask the manufacturer to change things to suit what you want.
Great answer Teabag. Vega my only comment is that this is something that you will get to know about with experience.
When I got my first CT I was as green as, oh yeah I had been tenting it for many years but a CT was a whole new ball game for me.
I bought a pre loved one that fortunately was a true 'off road' CT (no requirements for change required there) and then proceded to fine tune it over time with options that suited me. New stainless steel water tank, electric tap and kitchen were just a few examples.
After a couple of years I discovered I knew so much more about CTs than I ever thought possible but it did take first hand 'experiences' and chatting with other CT owners to achieve this.
We can all tell you 'our requirements' but ultimately it comes back to your needs. Like Teabag said, research, research, research and chat to fellow CT owners.
Every time you are out and about and see somone, stop and get them to tell you about their CT. The majority of CT owners are only too happy to show and tell.
After you are armed with as much info as possible, then I would approach the manufacturers and I would talk to all of them irrespective of which one you may be leaning towards.
You'll be surprised at what you bits of info you'll pick up along the way, you'll also surprise yourself as to how knowlegable you will have become.
Hope this helps and GOOD LUCK...