High-fee but stable PSP vs. Cheaper but problematic – Which to choose for large flows?

Looking for a new PSP for onboarding clients with a fairly large transaction flow, millions of dollars per month. I found one with higher fees but a good reputation for stability and issue resolution. Another is considerably cheaper but has feedback about system outages and withdrawal delays. I'm undecided on which way to go. Anyone with experience in heavy flow trading, please share your thoughts on whether reliability or fees are more important in the long run.