TL;DR. Two $0 cash-back cards with different rotating-category logic. Discover it earns 5% on rotating quarterly categories (capped $1,500/quarter, $75/quarter max) plus Cashback Match (doubles all first-year earnings). Custom Cash earns 5% on your top monthly category (capped $500/month, $25/month max). Discover wins in year one because of the Match. Custom Cash wins long-term because of more flexible category selection.
The three dimensions that actually decide it. First, first-year bonus. Discover's Cashback Match effectively doubles year-one earnings (no upfront welcome bonus needed). Custom Cash has a flat $200 welcome bonus. For a high spender, Discover's Match can exceed $400. Second, category mechanics. Discover's quarterly categories are pre-announced and rotate (gas, groceries, restaurants, Amazon, etc.). Custom Cash auto-detects your top monthly category from a broad list. Third, cap. Discover's $1,500/quarter is higher per quarter than Custom Cash's $500/month.
Real customer scenario for each. If you have $5,000 in projected first-year card spend, Discover's Cashback Match doubles year-one earnings, often netting $300-600 in year-one cash back. If instead you want a long-term card with consistent 5% on your dominant category, Custom Cash auto-adapts.
The trap to avoid. Holding Discover beyond year one expecting the match to continue. The Cashback Match is first year only. Year two onward, Discover is a 5% rotating-category card with the same caps; not bad, but not differentiated.