If your goal is to learn to use a typical SLE kite on water, then using a trainer first is a great idea. Seems the schools pooh-pooh the need for this -- more like 20 minutes on the trainer, then onto bigger kite with short lines, then into the water, up and riding by 3rd day. That's great, but you haven't really been fully trained in what a kite can do, the problems it can experience in varied conditions, and how all the safety systems work.
My kite mentor advised me to do a minimum of 20 hours on the trainer before doing lessons. Learn how to self-launch, fly it without looking, with your eyes closed, pick up a stick and throw it or tie your shoes without losing control, run down the beach with it in powered position, let go of the bar and see what happens, release your safety see what happens and learn to reattach it, learn to self-rescue on land and in water, practice water relaunches, try looping kite and how to unswivel bar, practice power sweeps and see how far you can drag your feet in the sand, sit on your butt or kneel and do power sweep up to your feet. Then you can do downwind body dragging, even with a 2m.
With all that done, when you do your first lesson you will probably be able to do your first body dragging session without ever crashing the kite. And when you finally go solo, you'll have an indepth understanding of the safety systems, not have to be afraid. Major climb up the learning curve!
The 2-line trainers are ok for learning how the kite flies and how to hot launch, but you'll outgrow it in 2-3 sessions.
The 4-line trainer simulates a full-size SLE kite, especially if you use a full-size bar with all the normal safeties and connections. For example, this is the one I have:
http://www.oceanrodeo.com/products/rise_2m_sle_trainer It is a scaled down version of OR's Rise. Buy one used for $200 then sell it to the next newbie for $200. Worth every penny!
Good luck!