Any pros/cons between svn vs portsnap?

The added bonus of using SVN instead of portsnap(8) is that it's easier to merge any local changes and/or revert changes on a single port. If you're not in the habit of modifying ports or need to change versions you're probably better off using portsnap(8). For most people portsnap(8) is easier to use and they don't need the added functionality of SVN.
 
e.g. if I modified the make file for sysutils/tmux, running svn update will restore the original one. Is my understanding correct?

Thank you

No, svn update will merge your changes with the changes from the upstream repository. This is what all revision control systems do by default on update operation. If you do svn checkout you'll lose your local changes.
 
Does portsnap correspond to the new (i.e., 10.2) "quarterly" default in pkg.conf, or will the ports that portsnap downloads be out of sync with pkg's default?
 
To add what SirDice mentioned, think of the ports tree as a rolling release. Updates to the ports tree are committed perpetually. The official package repositories are a snapshot of the ports tree at any given time. For quarterly packages(which are now the default), they are built from a snapshot of the ports tree every 3 months or so. For the latest packages(which was the default prior to 10.2-RELEASE) are built every other day or so. With that in mind, yes, the packages will always be out of sync with the ports tree. This is one of the reasons it is recommended not to use both ports and packages at the same time unless you really know what your doing.
 
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