The biggest difference between QMF and SPUFI is that QMF is a query/reporting environment with the ability to format reports. SPUFI is just a quick and dirty SQL execution engine. If you need to produce nice looking reports, enable user input to a query, or store your queries and reports for future usage, QMF is a much better technology for doing so.
Keep in mind, though, that SPUFI comes for free with DB2 whereas QMF is an add-on product and costs money. Not every DB2 customer will have QMF, whereas every DB2 (mainframe) customer will have SPUFI.