Wear what you are most comfortable in. I only wore a 3mm wet suit and was totally fine for my dives, water was about 74F which is WARM for me!
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---------- Post added September 23rd, 2013 at 08:29 AM ----------
Also: congrats on your decision. Seriously! Cavern/Intro was the most grueling, intensive course of my life....but I've never grown more as a diver before or since. Do you mind me asking what instructor and setup you're using?
Thanks! I am really looking forward to the training and I know it won't be a walk in the park My instructor will be Harry Gust who seems to be absolutely unknown in this board but has a very good reputation over here in Germany. I hope this will be a good decision to go with him in the end. I use a backmount rig as I got some training with a DIR background and I don't want to switch to anything that is unknown to me in addition to the cave training.
As for the instructor, I'm not familiar with him but I know someone who might be....I'll ask around.
About your choice for Backmount: I'm a huge sidemount proponent, but I'm a bigger proponent of diving what you're comfortable with. I'm glad you're not changing anything. Overhead will be so different that switching anything will just add to the stress. If you're comfortable in doubles, dive doubles. You can add sidemount later on if you want to. If so, and if you want to do it in MX....I've got an awesome instructor to recommend.