I'm not sure how fimilar you are with Crystal.
The most important question is your datasource.
Are the fields you'll be using already in a company database? If so then using crystal would be easier then word because within MS word you would have to create your own datasource for storing people's name, address, zip, etc and then you create your letter.
Some people would insert a subreport for the mail merge. Kind of like if the mail merge is something like a packing letter that can include the products ordered with a total. Subreports can be tricky when your trying to link it. All depends on how the tables are setup.