When you introduce a second ISP and want to do load balancing it is frequently helpful to think of it in terms of how you balance outgoing traffic and how you balance incoming traffic. In considering incoming traffic it is helpful to know how much of the incoming traffic is in response to requests initiated from inside your network and how much of the traffic is initiated from the Internet to resources in your network. Do you know how much and what type of traffic is initiated from the Internet to resources inside your network?
For a larger company using 2 ISPs we might look at running BGP as a way to achieve load balancing. But it sounds like that might not be a good fit in your situation. So we should look for other options to achieve load balancing for you. Unfortunately the ASA5505 does not provide many options for that. Perhaps you can have a default route going out an interface with the primary ISP and have static routes for certain network going out another interface to the other ISP?