弹性 (tánxìng) refers to elasticity in materials or flexibility in policies/thinking — the ability to recover shape or adapt. 韧性 (rènxìng) is toughness: the capacity to resist deformation or breakage, often used for materials like metal and for resilience of character. While 弹性 emphasizes being pliable and springy, 韧性 emphasizes being strong and durable.