Keeping luggage overnight with UA will increase the chance of lost luggage. Your bags will be sent to UA by AC, but once at UA there will not be a storage belt/container/area assigned to your UA6290 flight until the next morning. Consequently this increases the chance of lost bags as the bags do not have a designated storage locations.

My recommendation would have the bags checked to LAX and then recheck in the morning for the trip up to MRY.