Which frequent flyer program is better if flying from LA to Canada - Velocity or Aeroplan?

If travelling from Los Angeles to Canada is it best to sign up with Aeroplan for reward points or is it better to use Velocity.?

That depends who you are flying with. It sounds like you are booked with Air Canada? If so, Velocity will earn you no points as Virgin Australia and Air Canada aren’t partners.

You could however sign up with Singapore Airlines KrisFlyer, and credit your miles from that flight there (if the fare booked is eligible for earning miles). You can then transfer them to Velocity from KrisFlyer.