John
As already stated by lots of folks on the list, this is largely a business decision rather than a technical one. However, there are some more useful thoughts:
1. Is the decision to multi-home consistent with your other redundancy plans?
For example, why go through all the trouble of multi-homing and setting up BGP, only for both circuits to be plugged into the same router? ..or, two routers but neither of them on UPS.
This is akin to insisting on a Class A bank-grade firewall but not bothering to put a lock on the server room door...
2. Multi-homing is usually considered critical when one is discussing hosting of some kind. Could you be served with multiple servers in geographically separate collocation centers inside one ASN?
While many MIS departments like to have direct access to their own servers, this can often be an emotional preference rather than a technical one. Often only the "public facing" servers need BGP redundancy. The back-ends can be set up to fail-over to separate VPN/IPs in separate ASNs.
Having said all that, I prefer physical access to my machines too. So I'm a hypocrite.
3. If you are not doing hosting, a two-ISP NAT solution may make more sense than BGP. In addition to burdening the global routing tables; good BGP management is expensive. It involves either hiring someone with the proper expertise/experience or purchasing that expertise. Relatively speaking, there are not a lot good experienced BGP admins out there.
4. What is the price of downtime, in real dollars? For many business, this really can be estimated. Consider lost time (wages, utilities, etc.) and lost sales. Then compare it to the various options.
Just my two cents,
John