I have used DD-WRT for a long time, I still have a pair of DD-WRT's and one, up until my ESXi setup, was hosting my second WAN IP address. The other one is configured as an emergency backup for my main LAN setup just in case my Sophos takes a crap while I'm on travel. DD-WRT is pretty good in my book. I have never tried Open-WRT.
DD-WRT isn't bad, but OpenWRT is much better if OpenWRT is available for your router. While DD-WRT is customizable to a point, it will never be to the extent that OpenWRT is, as OpenWRT provides hundreds and hundreds of packages that can be searched for and installed, either via cli or the Web Management GUI. Once one tries OpenWRT, they'll almost immediately see the major differences and benefits it has over DD-WRT.
I use OpenWRT on my WRT1900ac and WRT1200ac, with DD-WRT running on my R6300 v1 (unfortunately, while the R6300 v2 has OpenWRT support, the v1 does not). I prefer to build my own OpenWRT images, as you can build all the packages you want into the image (as well as ensuring all your /etc/config files are customized to your environment, as well as any other files, scripts, configs, or certs you want in the image), versus having to manually install them if you utilize the stable or trunk images (trunk is currently on kernel 4.4.7 or 4.4.8 [one can also utilize kernel 4.1.x], with CC stable utilizing 3.2 something if IIRC) . If you do choose to try OpenWRT, ensure you have a USB-TTL cable in case you brick and need to perform a TFTP flash.