eBGP With Two ISPs

We have two datacenters in two different states. Each has one ISP at the moment. We are already doing eBGP with one of them and will be changing to BGP (from static) with the other when we get the second ISP. Lets assume for simplicity sake that each datacenter has an ISP1 and ISP2. We want mission critical traffic (to our core platform) to route in/out of the internet using ISP1 while all other traffic should route in/out over ISP2. Lets focus on datacenter1. At datacenter1 we have a public address space with a size of /24 and at datacenter2 we have a /26. I was thinking to myself, yeah we could advertise a /32 out ISP1 so that this is more preferred from the internets perspective and would win. This would allow traffic into our org from the internet to take ISP1. I have a feeling that the ISP won't allow this though. My next way of accomplishing the desired behavior would be to get a second block of addresses at each location and out ISP1 make them preferred via BGP selection process (local pref or AS-Path prepending). The new block of addresses would also be advertised out ISP2 but less preferred. For the reverse (traffic out of the org), I suppose I'd have to find out what IPs our mission critical app connects to and have the ISP1 provider advertise those blocks to us. This is what I'm thinking of doing but how would you accomplish it?