The basic difference concerns your set of choices. With an open choice policy you are free to choose any appropriate treatment provider, while with an open access policy your choice is restricted to those treatment providers operating within the insurance companies "network".
Here is a description of how Aetna's open choice policy operates:
Participants can choose, if they wish, to go to a doctor who is in-network and pay $15 per visit to a PCP or $20 per visit to a specialist or treatment without any further need to file a claim with Aetna. Alternatively, participants may elect to receive treatment from any physician not in the network and obtain reimbursement by filing a claim with Aetna, subject to the annual deductible, the normal co-insurance and subject to the providers' fees falling within reasonable and customary norms.
Source: (of all places) United Nations site (mirror).
On the other hand, an open access policy restricts your choice to "in-network" treatment providers. In other words, your policy gives you "access" to their network.