Short Answer
We offer both because they are both successful. Each class will have a different dynamic and structure but work towards very similar goals.
The More You Know
If the child is just starting out it's personal preference as some parents don't want to be in the water and for others its important to start in the water with their child. Or they can always start in LS and transition to Semi, but not vice versa and explain that a Semi is a commitment until they're 3, no flip-flopping.
Some parents know from seeing their kids with babysitters/teachers/grandparents/other authority figures that their kids listen better to other adults, or vice versa and know that they do well with the parent.
Or if they start in LS and the instructor thinks your child could benefit from a semi then they might recommend that, or advise the parent to ask the instructor since they see the child in the water and can properly assess.
Lastly, talk about the differences in more detail, the biggest one is parent not being in the water for semi, but also explain that both levels are about learning through play and building trust and focus on comfort in the water, both great for toddlers and both teach backfloating, rolling over, etc..