We just inherited a client using a FortiGate 60E device running OS 6.0.6. For this device I see I can upgrade to the latest 6.0.x which looks to still be maintained but also to 6.2.x, 6.4.x, and 7.0.x. Which would be considered the golden/stable branch? I would Assume 7.0.x is out of the question for this old unit
I wouldn't assume that there will be any issues with v7.0.x in this box just because of the hardware. Now, when it comes to which version/branch is better, it is always a hard question because it really depends on the kind of features you use, etc.
I always try to stay on a branch that is "mature enough" because they tend to be more stable. However, new features and bug fixes may "force" you to install newer versions.
Latest version of v6.4.x (v6.4.8) seems to me like a good option at the moment... I am personally running a FWF60E at home in v6.4.6 without any issues.