Much wider hardware support with OpenWRT. If the hardware is accessible and supports Linux, there is an OpenWRT target for it.
There’s even a x86 and a raspberry target.
If you’re happy with DD-WRT, no pressing need to change it.
But if you want more features or specific packages, OpenWRT is the thing to look at.
openwrt is the more serious/upstream friendly distribution that is maintained and updated like traditional linux distributions. DD-WRT may support some abandonware like broadcom wifi devices.
I've been using FreshTomato, which I got pre-installed from a seller on ebay, but IIRC DD-WRT has better support for some Broadcom chips because they allow binary blobs which have better hardware support.