Two crews can charge the same monthly rate for a yard and leave behind completely different properties three years later. Here is where the difference is made.
The four year degree is treated as the safe choice and the thing to do while undecided. It is a large commitment and it deserves to be chosen rather than defaulted into.
Retention money usually gets spent on whichever lever is easiest to pull. The options differ enormously in how long the effect lasts and what it actually costs.