The selection of FOS is usually based on the requirements or features you are going to use.
The latest version is 7.2
I would suggest going through the release notes that highlight any known issues. If there are known issues that are pointing at the feature you are going to use on the FortiGate, you can check if this is resolved in the next available version.
Just to throw my 2 cents in as well. You will also see next to the firmware, under release type, Feature or Mature for 7.0 and 7.2 code base. There is no Mature release yet for 7.2; whereas there is for 7.0; and like others have mentioned, that is 7.0.9. I think they started this Mature and Feature designation with 7.2, but since 7.0 is now up to rev 9, it is classified Mature.
TheFeaturetag indicates that the firmware release includes new features.
"A release that contains new features also brings with it a greater chance of new bugs. Therefore, these releases are less suitable for use on production systems. Mature releases, on the other hand, contain more bug fixes and rather fewer new features and are therefore also to be regarded as more stable."--some blog I read.
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