A high performance router is must when it comes to provide a large size subnet, and could route the packet to external network. When it comes to Router along with Firewall capabilities then it becomes a complex tricky problem.
Simple routing solution can be provided with Cisco router, FortiNet router, in fact we found Edge routers manufactured by Ubiquiti is pretty awesome, provides a great throughput, and Edge comes in a very very low cost with only single time of investment. But when we also want firewall capabilities then it becomes difficult to work with Edge, because now you need a protection, QoS, shaping, filters etc..
A Dual WAN router is must when you want to support a fail over mechanism. If one ISP goes down, other will take it over in Dual WAN routers but generally auto-fail-over is not pre-configured in FortiGate as well in Edge, I guess it doesn’t come automatically configured which is quite a strange thing. Anyways be careful to configure the auto-fail-over mechanism in router for sure.
What is auto-fail-over?
Generally fail over is configured automatically, but this only means if you take out one ISP cable out of the router then other cable will start routing the packet, which means internet will work. But then scalaing is very difficult because when one ISP will go down then you may not be available to go physically and remove the cable, so we need auto-fail-over mechanism. It is possible in FortiGate for sure, and I believe it is also possible in Edge (which is currently not there).
When there are more than one connection available to you, you can easily choose to do some load balancing, means routing the packet through different ISP routes. Typically Load balancing is good if you have more than one ISP connections with similar latency and performance, because if they are different end use would face a kind of fluctuation or lag in frequent use of Internet, so choose it wisely; actually you can ignore this feature, rather use spill over.
When one ISP line bandwidth is entirely consumed then you can choose spill over mechanism to route the packet instead of dropping the packet for waiting for the turn. In many cases this will turn out to be a very good choice, specially one sudden day there is huge traffic then using secondary ISP to route packet is a very good choice, instead of directly increasing the bandwidth. If bandwidth consumption is consistently high then possibly bandwidth need to be increased.