Moving with GSM and a follow up with COBRAS will work fine and will allow you to move groups of users while both systems are up and running as you go.
DiRT will get a bit more data (COS objects, templates, name lookup handlers of note) - but it requires the same version as the backup and it does a complete DB replacement.
More folks are using GSM or GSM with COBRAS follow up of late than DiRT for this reason - smaller sites that don't have a lot of handlers and don't use notification devices etc... just use GSM alone. A COBRAS follow up will get all your call handlers, schedules, interviewers, extra subscriber data and distribution lists as well so if those things are important to you, it's worth the extra step.