The "RPG-ness" of GBC vs GBA is pretty comparable. You take your character and rise through the ranks in a more grounded golf/Tennis centric setting, finishing off with a more Mario focused finale. The mechanics of each game match their respective console games, so the GBC games are a bit more vanilla while Golf GBA let's you put spin on your swings and Tennis GBA adds power shots. Additionally, Golf GBA has both singles and doubles golf, unlike Golf GBC. Tennis has singles and doubles in both games.
What is notable is that the GBA games are story sequels to the GBC games. It's nothing too impactful - they take place in the same settings, bring back other characters, but it's not essential to play both unless you really like one and want to play the other. In fact, most of the golf courses from GBC return in GBA with the same layouts.
One thing you might want to consider is connectivity between the console and handheld versions of each game. While they don't affect the RPG campaign, you can import your character and use them for all of those games' modes as well as unlock content:
- Mario Golf (N64 and GBC) simply lets you transfer the created characters to N64 and back.
- Mario Tennis (N64 and GBC) have a bevy of unlockables between the two - nore Mario characters and minigames in GBC, courts in N64 awarded when you do well in said minigames. This is in addition to the transferable characters.
- Mario Golf: Advance Tour gets extra Mario characters when you connect it to Mario Golf: Toadstool Tour, alongside transferring your characters.
- Mario Tennis: Power Tour cannot connect to Mario Power Tennis at all. No unlockables, no transferring characters. Both are completely stand-alone titles.