My strong recommendation is to purchase another external modem and connect each modem to the console port of each router for remote management of the router.
The two options you are contemplating don't provide console access in the event the router cannot load its configuration. If the router is sitting at the ROMMON prompt or rebooting itself continously you will not have remote access to its console to firefight.
Here is the URL to configure a US Robotics modem for the console port of a router.
http://www.cisco.com/en/US/tech/tk801/tk36/technologies_tech_note09186a00800941c9.shtml