One's a strategy RPG with giant robots, the other's an RPG with small robots.
Completely different, really.
Anyway, SRW:J is one of the easier ones, has some very nice animations considering that it's on the GBA, and has Tekkaman Blade in it, so it seems like a good place to start.
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xfd dude, it's just a popularity contest on the internets.
-Vlado