Does this particular router have two WAN interfaces to which you are plugging in?
With regards to load balancing and failover, you have a few scenarios with single points of failure.
1. Spoke Router failure
2. ISP Failure
From experience, a majority of the problems occur at the ISP level, so I would recommend a diverse link if possible from another ISP.
Speaking on load balancing, there are a few gotchas which you may not have considered.
1. If one ISP is experiencing issues with their service (ie. packet loss, high latency, etc) it will make troubleshooting much more difficult to isolate the source of the problem. In addition, it will also make the entire site link look terrible since half of the traffic will be travelling over the degraded link.
You will also need to consider not just load balancing from the spoke, but also the return path through the hub(s). By leaving equal cost routes everywhere (ie. equal cost from your head office to the hubs, then equal cost from both hubs to the spoke) if there are any problems with a particular link/router you will affect your entire deployment as you may end up with asymetric routing (going out via Link A from the spoke to the hub, but returning via Link B from Hub to Spoke)
If failover is your primary option, my personal opinion would be to split your sites into two groups, and make one group use hub A as the primary with B as the failover, with the second group using the opposite. This makes traffic flow very deterministic and decreases the time to isolate where your problem is as it's quite evident when all the sites in 'groupA' are having issues...
Cisco has a very comprehensive (but looong) read on this here. What you're probably looking for is the Dual Hub - Single DMVPN Layout which gives you what you're looking for. However, I'm not a fan of asymetric routing, which the Dual Hub - Dual DMVPN Layout will solve.
http://www.cisco.com/en/US/tech/tk583/tk372/technologies_white_paper09186a008018983e.shtml#dualhubsingle
