I use and enjoy both XMPie and FusionPro. Each one has strengths where the other has weaknesses. For example, XMPie's uCreate plugin for InDesign allows one to create the complete variable document inside InDesign, taking full advantage of InDesign's styles, type engine, end everything else. FusionPro's plugin only allows you to define specific frames as either variable text, image or chart. You then use the plugin's export feature to generate a variable PDF. From there, you have to create all of the rules and complete the merge from within Acrobat. Once you have handed the merge over from InDesign to Acrobat, any layout changes require a fresh export from InDesign's FusionPro plugin, but at least it is smart enough to avoid overwriting your variable components and other settings that you have configured in Acrobat. FusionPro has made that process relatively painless.
Of course, Acrobat obviously doesn't have the type engine, styles or flexibility of InDesign, but it is a solid merging platform nonetheless. Where FusionPro really shines above XMPie is in the actual merge process. XMPie's uCreate imposition options are atrocious and poorly executed. Unless you're running XMPie Server, you have to manually set up imposition specifications for every job, and it tends to crash if you have the wrong PDF export preset selected (and the pdf export preset changes to match whatever your last pdf export used, so you always have to babysit this setting.) FusionPro elegantly handles the imposition aspect of your merge by coming with a light weight standalone imposition utility that you use to define how you want the merge to flow. You then save the setting as a file that can be applied to any other FusionPro merge you like. Both XMPie and FusionPro support cut stacks and N-UP, but FusionPro allows you to merge to batches of files where XMPie uCreate does not (need the server for that). You can set the number of records desired for each file, then it will automatically export PDFs that contain only that number of records. This is easy to appreciate when you have to do a 30K record merge, and don't want to choke your RIP with a gigantic file that could literally take days to process. Bite-sized files via XMPie require you to manually enter ranges of records, process that range, then manually enter the next range etc.
I haven't used FusionPro's online functionality, but I use XMPie's uStore and uProduce almost daily. It's a pretty convoluted endeavor at this point. XMPie's back end has been engineered by developers who, historically, have handed off their product to corporations with the resources to assign their programmers and developers to "make it work." Their cost has been prohibitive for smaller business that can't afford to do that until quite recently. As a result, their functionality has a lot of holes and gaps that seem ridiculous to someone like me who has worked in the print industry for 25 years. Don't get me wrong, it's pretty solid as long as you don't want anything terribly pretty or fancy. If you do need something outside of what comes in the box, it's all on you.
I hope this helps.