Omastar has a better defense and better SpAtk
For ingame This is the one I would go for as he can withstand most hits with better defensive stats than Kabutops.
This is a set suggested by trachy,
Omastar (M) @ Focus Sash
Trait: Weak Armor
EVs: 36 HP / 252 SAtk / 220 Spd
Timid Nature (+Spd, -Atk)
Scald Ice Beam Shell Smash Hidden Power [Rock]
I find it kind of odd so here's another
Trait: Shell armor
EVs: 252 HP / 4 SAtk / 252 SDef
Modest Nature (+SAtk, -Atk)
Stealth Rock Spikes Toxic Spikes Surf
The item for this moveset was red card. Meh any item will work ingame really.
Either one works as a fire wall as they are both 4X resistant to fire moves.
If you want a slightly speedier Pokemon who relies on Attack then go for Kabutops but this is my suggestion. Hope it helps FF!!