Gargoyle is easier to use for both new users and advanced users alike, much more stable, and in my opinion, a much friendlier community. It can easily be extended to suit your needs. With Gargoyle (and OpenWRT, the system Gargoyle is built on), things just work the way you'd expect. Full source code is released alongside the compiled firmware. Most settings explain themselves, and in the few times they don't, the included help is up to date. Two weeks uptime so far, and the only reason it was down was to fix a setting I screwed up myself (did so by manually editing config files, should have just used uci to handle it). With OpenWRT, guest networks are pretty easy to set up too.
With dd-wrt, if you want to do anything not included in the firmware itself, optware is your only option as they never release full source code anymore, meaning you can't build anything useful. Settings that worked 10 minutes ago might stop working after you reboot (happened to me too many times to count...). It also tends to crash and reboot quite frequently (runs about 6 minutes on my router before either hanging or rebooting). On the plus side, if you can keep your router happy with dd-wrt, and you have enough flash and ram, you can easily set up enterprise security if you need it. Guest networks are also easy to set up, if the wireless interface will play nicely with the options dd-wrt tries to force (pretty likely, but sometimes it doesn't)
I know I'm biased, I want something that just works, and dd-wrt never did with any of my better routers. My DIR-615 version D1 worked better with dd-wrt, but my WZR-600DHP (that SHIPS with dd-wrt) can't run it right, but OpenWRT and Gargoyle are rock solid with it. I think dd-wrt's problem is half-baked features and drivers getting out as main releases, where as both OpenWRT and Gargoyle get everything as complete as possible before releasing it, and at least make sure there's an easy fix for anything that might go wrong. OpenWRT is less user friendly than Gargoyle, but not painfully so. After awhile, it's just as easy to use either one.
I hope this helps
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