Just from glancing over the specs, it looks like the 831 has more features: Dynamic DNS, IDS, a DMZ port, the ability to do web filtering via websense or N2H2, hardware-accelerated encryption, EasyVPN (which is nice to have should ever want to VPN back into your home network - it makes setup really easy).
I think the SOHO 91 is an older model. It's probably on it's way to being phased out. I'd say the 831 looks like a good choice. I guess the question is: do you need Cisco? There are a lot of other cheaper routers out there for home use. The Linksys RV series are pretty good. But if you need Cisco, the 831 looks pretty good.
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