The importer built into XF 1.5 is going to be considered more stable for a period of time. The import code in XF2 is brand new and the implementation of the actual importers will take time (though they are in development as we speak) and they will be mostly untested.
As much as we would appreciate people doing test and live imports once they are available, if you're mostly concerned with having a quicker and more hassle free import process, importing into XF 1.5 first and then upgrading to 2.0 is the recommended action.
Once you have done the import into 1.5, we would recommend making a back up of that and then doing test upgrades on that backed up 1.5 installation.