Is there a difference in support quality between CSP-provided support and buying Microsoft Unified Support directly?

They're different models rather than strictly better/worse — Microsoft Unified Support is Microsoft's own enterprise-level support offering, typically used by larger organizations wanting a direct relationship with Microsoft. CSP partner support is provided by the partner (sometimes with their own specialized expertise or faster personal response), with Microsoft escalation available when needed.

For many small-to-mid-sized organizations, a good CSP partner's bundled support can be more responsive and personal day-to-day than a large-scale enterprise support contract, but this really depends on the specific partner's quality and your organization's needs.