tl;dr: With more realistic models with diverse length and types there is no clear winner. With Allocations Utf8Json wins.
Input: The models from Jil.Benchmark: https://github.com/kevin-montrose/Jil/tree/master/Benchmark/Models and ported it (with a few changes) to Benchmark.NET https://github.com/Tornhoof/Jil/tree/master/Benchmark
BenchmarkDotNet=v0.10.9, OS=Windows 10 Redstone 2 (10.0.15063)
Processor=Intel Core i7-4790K CPU 4.00GHz (Haswell), ProcessorCount=8
Frequency=3906248 Hz, Resolution=256.0001 ns, Timer=TSC
[Host] : .NET Framework 4.7 (CLR 4.0.30319.42000), 64bit RyuJIT-v4.7.2110.0