Which servant rolling next?

Don't think you can go wrong with either of them. Raikou comes with the added bonus of getting the event CE's if that's important to you.

If I were u, I will aim for support servants like waver, tamamo or merlin. Getting merlin is the best case scenario as it is the most op servant. However, not getting merlin is still ok since it will be a welfare servant (like jalter) if u know what i mean. If u like certain servant, feel free to go for it. The only rule u need to remember is: gacha4waifu xD

Out of Scathach, Raikou, and Sanzang, I think Sanzang will be added to story pulls so you can get her randomly whenever, Scathach will have another solo banner in a year or so, and I'm not sure about Raikou. In terms of who's the best out of those 3, Scathach is supposed to fall off as the bosses get more health and Sanzang is not the best caster considering there's Merlin and Waver so I think Raikou might be strongest.

Both scathach and raikou should have 4 or 5 rate ups in the future (though they may take some time to come back).
If you want just pure damage, you can't go wrong with Raikou, but you already have Jalter, so damage shouldn't be a big problem. Scathach offers an awesome targetable quick up and guaranteed stun on her NP, in terms of utility she's better than Raikou. Since you already have a strong DPS i'd go for Scathach.

But as already pointed out the best option would be to aim for a strong support, since the game will always be gacha4waifu, go with the one you like the most.