I have both in Diamond. Honestly, there's no doubt in my mind Scarlet is the better pick in most situations, especially post Eliza toolkit buff. Eliza now has Heal Block and Curse built in, which leaves the only tools unique to Tomb being Evade and Hex. Neither of these are bad, but the combination of easy bleed/enrage and naturally lower damage (9.9k vs 7.3k) means even if you have a high level Fury the enrages are doing less for you. Scarlet's primary weakness is mitigated by having a taunt equipted on her to override the bleeds once you knockdown the opponent.
Tomb has one two things going for her: move flexibility and safety. Thanks to her built in debuffing, you aren't locked into using Sekmet Bite to curse, or Turn to heal block (but Fury is mandatory for haste and enrage still). This means you can use her BB2s and deal immense damage while still getting a random debuff that might help you out. You can use Nekbet Breaker and Khepri Sun and still potentially get the debuff you want. Compared to Scarlet who has to run 3 Sekmet BB1s and a tuant, she has lots of move flexibility while Scarlet only gets 1 flex slot. On top of that, The combination of her Evade, higher HP, and not hurting herself also gives Tomb more freedom to try and punish. You can use the evades to get you knockdown loops and Wither will hopefully prevent reversal BBs with Death Mark letting your damage be somewhat reasonable. Unfortunately, this means you're still relying on RNG with tomb to be competitive. Scarlet on the other hand is consistently lethal and quick thanks to the powerful tools her SA gives her on top of her much higher attack.
Honestly, I'd only recommend Scarlet for diamond once you've burned through the strong fire options (Harley, Mean One, Dragon Brawler, and thrill are the big four imo, with Parasite also being solid). In that time you'll also likely be able to make Bloodbath (if you get her) and Diva (I like Diva better for rift, less for PF). She's a worthwhile gold however, since it's not too resource heavy to get there and she can longshot very well in PF. Tomb can work in gold since Eliza isn't exactly loaded with low rarity prizes, but so much of dark beats her I don't think I could ever recommend her for diamond if you're looking for always the best choice. The list of darks I personally think rank lower than her are Gray Matter (Although if you make 3 she's a beast) and Decrypted. Mayyybe Stage Fright, but Doom + her MA is powerful utility PF wise.
Tl;Dr: Scarlet is much stronger, Tomb is weak but safe, neither are going to make the game free but Scarlet can hold her own in longshots with bleed and enrage while Tomb can be relatively safe with evade and flexible since she's not restricted in what she needs other than BBs.
