Lion
Lions offer what consumer researchers call consistent product delivery. A lion will be lion-shaped tomorrow. It will roar predictably. It will sleep for approximately 20 hours daily with admirable consistency. The Serengeti Reliability Institute reports that lions maintain 99.3% behavioural consistency across their adult lives, varying only slightly during mating season when they become, by all accounts, insufferable.
One always knows where one stands with a lion. Usually, at a safe distance. This predictability has made the lion a cornerstone of safari tourism, with guides able to guarantee sightings by simply locating the nearest shade and waiting.
Rainbow
The rainbow, conversely, operates with the reliability of British public transport. It may appear. It may not. Weather conditions must align with mathematical precision: the correct angle of sunlight (42 degrees for the primary arc), adequate moisture, and an observer positioned with the sun behind them. The Met Office Rainbow Prediction Unit admits to a success rate of merely 34% in forecasting rainbow occurrences.
Double rainbows, whilst spectacular, appear with even less predictability. Triple rainbows are so rare that the International Rainbow Documentation Society has recorded only 147 confirmed sightings since 1950.
VERDICT
The lion's 24/7 availability (assuming proximity to sub-Saharan Africa or a reputable zoo) decisively outperforms the rainbow's capricious scheduling. For sheer dependability, nothing beats a creature that will absolutely be sleeping under that acacia tree where you left it.