Both parks are official, purpose-built, and close enough that a ranking would be a personality test. Use facts.
You Yangs (Parks Victoria): over 50 km, two precincts, Kurrajong about 20 km of flatter loops, Stockyards more technical, dogs on lead, about an hour west, gates 7 am–8 pm, Turntable fills in summer.
Lysterfield (Parks Victoria): 24 km purpose-built MTB, 6 km lake circuit, 2006 Commonwealth Games course, no dogs, south-east foothills, summer parking treated as full when full.
Choose You Yangs if you want more signed kilometres and a clear beginner box. Choose Lysterfield if you live south-east, want a lake walk for non-riders, or care about the Games-course history. Choose the one you can reach before 10 am in January.
Guides: You Yangs map, Lysterfield trails. Tours: /you-yangs, /lysterfield-1-day.