If you're moving the entire server DiRT is the way to go - it's what its designed for. If you're moving some users in a bleed migration or part of a load ballancing effort or the like, COBRAS/GSM is the way to go. DiRT gets more data than COBRAS does, so in your scenario DiRT is going to be the better option.
Some people really, really hate doing the 4.x install, dirt restore and then upgrade. They'll opt to install 7.x and do the COBRAS import from 4.0(5) directly but this leaves some data on the table and requires some extra steps (deleting users off the original Unity server, waiting for replication to complete then import or use GSM to move and COBRAS as a follow up import).
As the author of both tools I'll stick to my guns and always suggest doing the 4.x install, DiRT restore and upgrade - it's what we designed it for and how we've tested.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...