This is a very good question!
I would suggest using the GPL or LGPL.
In this case, I think you can safely position your templates as a work of Software and not Content. Both licenses are very specific in stating that any modification of the program source must be redistributed with the same license. They also make no restrictions on the use of software for producing output in any form. The LGPL is a bit more liberal in allowing distribution with bits of non-free software.
Where it gets ambiguous is the question of weather or not the compiled PDF, PS or DVI file is a "compiled binary" of the source TeX file. In this case, you may want to make it very clear in your README file that you make no restriction on the output produced with the templates and only wish to apply the license to modified versions of the TeX template files. Alternatively, you might want to rewrite one of these licenses to suit this particular problem as they contain specific language that relates to the technical conventions of compiled software.
CC licenses are generally for output content like texts (PDFs), images, audio and films. Although they are inspired by Free and Open Source models, they are not designed to deal with this particular question of source vs. output. GPL, BSD, MIT and other FLOSS licenses are more specific on these questions, so I would suggest avoiding CC in this case.