On defense, use both, and RG greatly benefits from gear X. On offense use ST Han, and he greatly benefits from gear X. So depends on where in arena you struggle.
Han really needs the gear because he just isn't tanky enough to be really effective. RG has eHP forever but his taunt is passive only, so he won't be protecting anyone until it's almost too late to do anything to stop a QGJ assist, Rey or Leia from finishing off someone at just over half health (since pretty much every arena team has at least one of those).
Han at gear 10 isn't as great a jump but once he gets to 11 he can be tricky and hard to focus down with decent synergy. You really need to go all the way with him imo.
RG at any improvement is a jump but even at 9 he is still a royal pain.
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RG at G10 max gear: A brick wall that stuns and protects Rey.
But both together is effective.
For raids, gearing up ST Han is better.
rg with non dodge lead
missing humbling blow on st Han is a gamechanger. missing humbling blow on RG not so much.
RG at any improvement is a jump but even at 9 he is still a royal pain.
I use qgj lead with rey and gs in arena.
Unless you got a lot of B2 on your shard, then han/rg on one team is asking for people to target you