In my experience the best keystone for most Sej/Amumu style tanks is the one you didn't mention.
Grasp of the Undying is super strong on more offensive tanks since it procs off of your HP. When you have a lot of HP the mastery damage and heal get to be significant and since you don't die quickly it's not unreasonable to expect to proc it multiple times, especially since it gives you sustain to help with this.
Strength of Ages encourages tanks to power-farm the jungle like they would with Devourer. But tanky junglers, like Sej, Rek'sai, etc., tend to excel at early ganking.
Sej doesn't and Rek'sai isn't a tank.
Rek'sai's a fighter jungler at her core. She excels at early game pressure. This means the only tier 3 defence mastery you should ever consider on her is GotU. You honestly should probably stick to Fervor of Battle on her though, it's pretty much just the best keystone for anyone who can make good use of it.
Sej on the other hand has sub-par ganks pre 6. They're not Shyvana tier or anything but she certainly doesn't "excel at early ganking". She gets outperformed in early ganks by any traditional early pressure jungler, (hence the name) she picks up post 6 but she's still far worse at ganking than people like Lee and Elise.
Sej actually does play a farm game early. The only time you can really gank as Sej is when someone offers you the kill on a silver platter. You also do need to farm as Sej since she scales hard into the late game, her powercurve is very similar to a devourer jungler.
I'd say overall Strength of Ages synergieses fairly well with Sej's playstyle, definitely better than Bond of Stone. The reason why Bond of stone is better is because 300 HP just isn't that good compared to other keystone masteries.