1st of all I'm not sure why moderator moved this thread from Cisco forum to Networking forum since there is a question relating to Cisco router.
Moving forward, there are several factors in considering network device choice to run BGP. They are
* Budget or financial constraints
* Application throughput/performance requirement
* Future growth expectations
* High Availability expectation level
For 20 MBps pipe on each ISP link (with total ISP of two), you would probably prefer something like Cisco 3825, 3845, Catalyst 4503, or higher model.
As to failover scenario for a single Class C, your only feasible choice is probably Active/Standby between the two ISP although Active/Active is possible assuming you and your two ISP have proper BGP Community string.
FYI, Active/Active scenario convergence time is in general shorter than Active/Standby scenario convergence time. In other word, your customers should feel minimal or no interruption when accessing your website during one ISP outage if you have Active/Active scenario.
As of choosing ISP for BGP, I usually prefer Tier 1 ISP. Whenever possible, you should have direct BGP peering to Sprint and to AT&T. If this is not feasible choice, you need to make sure that both your ISP has direct BGP peering between each other.
Frankly, I never heard BGP peering connection via wireless. Since you are considering fiber link, it is most likely you will use the fiber link to deliver BGP peering and not using the wireless for the BGP peering.
With a 3rd ISP (Wireless ISP), you may be able to announce the subnet from the wireless ISP to the two previous ISP over the fiber link. Make sure that all three ISP are aware of it so they can do necessary adjustment on their end.