Sorry about the delay, as I was off line for a couple of days.
Append, just Adds the information from the unopened file to the file that you have opened.
Merge will attempt to merge the two files together. The biggest reason to Back Up BOTH files, is just in case things don't go as planned. If you do a CTRL+Z immediately after you have done the merge, it will Undo the merge. But, I suggest that Back Up just in case you start looking around the results and don't like what happened but for some reason, the UnDo doesn't work.
Also, remember that if you do the append, there is the feature to find duplicate persons.