I must admit I can't remember their payloads right now but I have wondered before why they bothered developing the SU-34 when, from an economies of scale view it might have made more sense to develop the SU-30 as the interdictor to replace the SU-24. After all, they have a ground attack capability.
They probably considered doing that but side by side seating was chosen because as a strike aircraft being able to get out of your seat and relieve yourself, and perhaps get a hot drink or light meal, but mostly the communication possible in a side by side seating arrangement was considered worth it.
The Su-34 design started as a replacement for the Su-24... as shown by its designation, and was intended to be a deep strike aircraft that reduced costs by not being a totally new design.
The Su-30 was actually just an Su-27UB that was intended for roles where two crew are more use than a single seater.
The original user of the Su-30 within the Russian AF was the PVO or air defence forces, who used it as a long range but slower MiG-31 with its big radar and ample weapons capacity... the MiG-31 rips out and tries to take down the bombers and attack aircraft at max distance while the Su-30 comes in behind looking for already launched standoff weapons and any enemy fighters.
The multirole versions of the Su-30 were for export... the Su-30MKI and Su-30MKK were for India and China respectively and were not really used by Russia, so for much of its career it has been a very basic simple two seat Su-27, but of course over time it got updates and upgrades and now it is being turned into a two seat Su-35 which should give it quite decent capability in both the air to air and air to ground role.
I suspect flying at very low altitude and at high speed the Su-34 would be rather better and able to carry a heavier payload further, but the Su-30 is certainly not a bad aircraft and will be faster than Su-34 at altitude and able to operate at higher altitude and also likely with better acceleration too, but both are able to perform fighter interceptor type functions.