They are two different series with different gameplay styles, although they were created by the same team.
For Growlanser, 2 & 3 came out together in the West as "Growlanser Generations". 4 came out via the PSP expanded port "Wayfarers of Time". And 5 came out on the PS2. That leaves 1 & 6 still untranslated.
IMO, the best is 4, on the PSP. I think it's an extremely overlooked game with a great battle system and a solid story. I'd say start with this one or 3, as they have self-contained stories. 2 is different from the rest as it doesn't have towns you can walk around in, just combat, story scenes and menus between battles. It's also a sequel to 1 taking place some years later so there are lots of references. I haven't played 5 yet.
Langrisser is solid, but more traditional SRPG gameplay, from the real creators of the genre (NCS/Masaya made SRPGs before Fire Emblem existed). There are lots of versions of 1 & 2 in English - 1 is on Genesis (as Warsong), there's a Korean PC remake with an English translation, and some mobile remakes. 2 is on Genesis, mobile, and there's an expanded remake on SNES called Der Langrisser with additional story routes (though it changes some things and is a lot easier). I think only one of the other games is translated, probably IV, and there two legit untranslated entries. I beat one version of each, solid stuff. Later games not developed by Career Team/Career Soft are generally considered awful.