I’m waiting for Atlantis to Summon, and then using my Epic Hero Tokens for Sand Empire — the question is timing.
If I get Tarlak or Ranvir from Atlantis, then I’ll use my EHT in July to have a shot at Seshat. Otherwise I’ll use them in June to try again for Ranvir.
So for me personally, the answer is both Atlantis and Sand Empire, but with different means — Atlantis Coins and probably gems for Atlantis, and EHT for Sand Empire.
I personally don’t think Sand Empire is particularly worth using gems on. The Appearance Rates for Seasonal Heroes are pretty abysmal, particularly for 5*.