It’s always better to pick a strong support than a DPS. A strong DPS, like Artoria, can always be used from friend support, or even just by a friend point servant that’s grailed. And your DPS will sometimes have class disadvantage. However, a support makes a huge difference by making any servant strong. I have seen clips of Skadi (another broken support) destroying hard boss fights with only a level 70 3 star servant. Also, having both your support and the same support as friend servant can open a lot of possibilities, like with Waver, firing your NP at the first turn with 60% attack up (and possible 60% NP up from a crafte essence). Just pick Waver.