I used both, and left DD-WRT about 2 years ago, due to problems with DHCP, QoS and instability. And I have not used it since I gave Tomato a chance. So I just read DD-WRT forum from time to time, but I have not tried anymore. So I can not say much about it, but I will tell my experience with Tomato.
Since I have VoIP in my house, I needed an effective QoS, and Tomato does that very well.
I would say that both has GUI, so I not all that hard to setup the router.
But I think Tomato's QoS is trickier to set, and the access restriction is way different to set compared to DD-WRT.
DD-WRT comes with more features than Tomato. But Tomato has many variants and mods with more features like DD-WRT.
If you are interested you can check
here.
And Tomato has its limitations like only 50 entries in DHCP static lease list, up to 50 QoS rules, 50 access retriction rules. But the a few mods take the limitations off.