The changes from the Sept vintage version 4s to the latest 5.x are really pretty small, and it's mostly in the sign up process (passwords instead of account keys.)
There may have been a few minor tweaks to the GUI, but nothing to make a real difference.
So if you don't plan to join another project, there is no need to upgrade, IMO.
Oh, there are slightly more informative messages sometimes, if the project implements them, but except for LHC being often out of work, your two projects rarely have problems.
Others may have different opinions on the 5.x GUI: since I use BoincView, I don't actually know much about the sequence of BOINC Manager interface changes.
If you do try another project at some point, it probably will be best to upgrade BOINC first: at some point they want to have all account creation through the password system, which requires 5.x
As far as problems with version 5, I've been using the release version 5.2.13 without any problems (Windows, I'm assuming here.) It's been the same since November 29, which is a fairly long time for a BOINC version, and I haven't seen any unusual complaints on the BOINC boards about this one, though you are right, some people reported problems with one or other of the earlier 5.2.x versions.
Whichever way you go, Happy crunching!