I did quite a lot of testing on this recently and there is no significant difference between LAN and OPT for throughput to/from WAN.
However I use OPT as my main connection here just because I also have it connected to an external switch and it makes changing VLANs on that link a lot easier.
You may have no VLANs now but if you have a 1G connection and a number of devices using it you'll probably want to start segregating them at some point.
Steve