In almost all cases two versions of Arbortext Editor can be co-installed. We are just starting to use 6.0 so our track record here is very short (and only a few users so far), but we have not encountered any problems yet. We are upgrading from 5.3, so I have no specific experience with 5.4/6.0.
The one time we ran into version trouble was when we moved from 5.2 to 5.3. Editor was fine, but the Documentum Adapter in 5.3 would not work until we completely uninstalled 5.2. We never chased down the exact problem or tried to "resolve" it so it is possible even that configuration could have been made to work.
If you haven't already, you should to join the Adepters mailing list where lots and lots of Arbortext users, admins, and developers hang out 24/7/366 (it is a leap year, you know!). Information on joining that list (as well as pointers to lots of other Arbortext resources) can be found here:
Is there a good way to determine if we should upgrade our version? I see a new 6.0 Arbortext and relate it to something akin to I'm using Word 95 when everyone else is using Word 2010. Also, there appears to be different version numbers of Arbortext and Styler running around? In my About Session, is Arbortext Styler 5.2 M130 referring to my Arbortext release or my Styler release? We haven't upgraded in proably (at least) two years, so I know I don't have the latest and greatest.
The short answer is, "No." The determination whether and when to upgrade can be difficult and may require input from many groups within your IT environment. Obviously if you work in a small deployment (let's say < 10 seats) and have a lot of freedom within your server environment (assuming you even have PE, the server component), it is much simpler than if you have 100s of seats and a rigid, process-bound (not judging!) production environment. That said, in either case (or the wide middle in between), upgrading costs time and incurs risk. Not upgrading, however, exposes you to increased compatibility issues as the rest of your environment moves forward (think Java, content management systems, browsers, etc.). If you choose not to upgrade long enough, you can run out of the PTC support period for your deployed version. Every major upgrade (5.2 to 5.3 for example ... not 5.2 m010 to 5.2 m020) is highly likely to require stylesheet modifications. Minor upgrades (m010 to m020) can but may not require stylesheet modifications. If you are using newer features, they may change a lot between minor versions. Older features are less likely to do so.
Finding the happy medium between upgrading too soon and waiting too long is an art.
In an ideal world, the longest time between upgrades would allow you to leave a version before it becomes unsupported, with enough cushion to handle the expected project calendar hiccups. The shortest upgrade cycle would be designed not to upset anyone in your environment while delivering new features (or increased stability, or some other Win) that have a demonstrable ROI.
Finally, an aside: Some deployments are so interdependent on other systems and/or so large and/or so cash-strapped, that they run "forever" on unsupported versions. The cost to keep a legacy system running well beyond all reason is, sometimes, not at all beyond reason. 😕 Imagine migrating many millions of pages worth of data while maintaining page fidelity (something PTC/Arbortext has never guaranteed within the product itself).