Currently, if I have FreeBSD 7.2-RELEASE amd64, which one of these Splunk install packages should I use?
Doesn't matter. One of them is a simple tar (the lower case one). The other is a BSD package that is installed with pkg_add.
You can run FreeBSD Splunk on FreeBSD v7.2, but you will need the FreeBSD v6.x compatibility libraries installed as well.
You need to run this for the libc.so.6
cd /usr/ports/misc/compat6x ; make install clean