I recently was hired as network admin. My experience is with HP Procurve. I currently have 26 ex3400 on version 15.1X53-D57.3 and the corrsponding J-web. Is it worth the time to jump to 18.2R3-S2 and t J-web 18.2A1? I have read that 19.* is not recommended. Would the GUI in j-web provide any beneifts?
The current recommended codes for EX3400s are 15.1X53-D591 and 18.2R3-S2. Versions prior to 15.1X53-D59 are already EOL. 15 versions after D59 have an extended support till Sept 2020 (this may change).
You can keep running 15.1X53-D57.3 if it is working fine for your network. But please note, that if JTAC support is needed, you may be ask to upgrade to an active release.
18 and 15 releases offer the same standard features, however, due to possible bugs or limitations detected by JTAC or under the PR database (https://prsearch.juniper.net/InfoCenter/index?page=prsearch) some customers require to move to 18 codes.
I would recommend keeping on the 15 code lines if there is not strict indication or necessity to move to 18 versions.
You could also check the release notes for JWEB version on the following link:
Hope that helps!
If that cleared your concerns, please mark it as solve!