- no, they are not interchangable, some software needs one or the other, see the README of the app you want to build for which one of the two is needed;
- it depends on what software you need;
- the conflicting header installed by both packages is /usr/include/event.h (and they are not the same file).
The only software now in the slackbuilds.org's repository that needs libev is i3: I know maybe it's not the best solution, but to avoid problems and install safely both, as I use sometimes i3 and lighttpd2 that need it and also other programs that needs libevent, I personally moved that header file from libev into /usr/include/libev/event.h
It creates no problem with i3 as it doesn't use that header (neither do lighttpd2), but can potentially create problems in the future with some other software depending on libev that include that: in this case, when building that app (but, I repeat, there are no cases that apply to the slackbuilds.org's repository now), I would simply add an additional CPPFLAGS="-I/usr/include/libev" to its build script to let it find the header in the new location.