It's always better to certify on the latest rather than the one that is about to become obsolete, unless you plan on only working for people who use SQL 2008.

The way I certified (way back when) was to build a small office / home office server at my house and install the developer version of SQL on it that I was trying to certify in. I grabbed a dozen different certification and just plain training books, ran through the exercises, then did everything I could to break the system. Each time I did, I had to learn to reinstall / fix it. That taught me more about SQL Server than anything else.

It also allowed me to work on a version of SQL that I didn't have available at work.

I can't recommend any 2012 materials yet because I have yet to touch that version. That's what I'm working up to now, though.