I believe that the older edition will be okay. There are still lots of older systems and tech out there.
Newer editions generally are just updates for newer tech that has taken hold and possibly may cover some aspects of up and coming technologies.
Older versions also tend to have (in my mind at least) better diagrams, photographs, editing, indexes etc. because someone probably really read the final product before publishing.
New editions tend to be "rushed" to market just on the basis of being Edition "X + 1".
Study from the old book to get the basics down. Then if there is a particular topic of interest or otherwise appears incomplete, google for more information on that topic.
At some point you will realize that the older...