TL;DR. A $95 card versus a $595 card. Sapphire Preferred is the right entry point for points beginners with low fee and Hyatt/United transfers. Strata Elite is for established points users who want premium earn rates on dining (6x) and hotels through Citi Travel (12x), plus 4 Admirals Club passes. They do not directly compete; Strata Elite is several tiers above CSP.
The three dimensions that actually decide it. First, fee. $500 difference. Strata Elite needs that much in additional value to compete. Second, transfer partners. CSP has Hyatt; Strata Elite (Citi ThankYou) has no Hyatt access. Third, lounge access. Strata Elite has 4 Admirals passes; CSP has none.
Real customer scenario for each. If you are new to points or want a single low-fee card that earns transferable points, CSP is the right answer. If instead you spend $15k a year on dining at the 6x rate, the Strata Elite earns 90,000 transferable ThankYou points on dining alone, worth $1,800 at 2 cpp, easily clearing the fee delta.
The trap to avoid. Comparing these as if they are in the same tier. CSP is an entry travel card. Strata Elite is a premium card. The right comparison for CSP is Venture, Autograph Journey, or Premium Rewards. The right comparison for Strata Elite is Amex Platinum or Sapphire Reserve.