MRM50... for the lowest possible tonnage, using MRM(40+10), thus, (12+3) = 15ton . Using MRM30+20 is 10+7 = 17 tons... I prefer to invest those 2ton in ammo or heat sinks! Remaining is trivial, 4xML for some jabs with low heat. Just for comparison, my GRF-5M(L) runs MRM40+10, 2MPL 4LMG and seems much more efficient than my Ares..
MRM40+10 generates 11.5+4+1.6 = 17.1 heat. MRM30+20 generates 9.5+6+0 = 15.5 heat. The heavier combination's lack of DHS would still be cooler in most cases, unless you chainfire the MRMs. Even then the MRM40 has longer cooldown. However, if you add the medium lasers, the added DHS will help to dissipate the lasers faster too, which could tip the build in favor of MRM40+10. I think there's something to be said for both cases You can run a LE280 at the same weight as a LE275 BTW (I added LFF because the slots were there and I can run an LE285 because of that)
Was gonna make the same comment about 30+20 being more heat efficient. I've said this before: there's rarely ever a reason to run an MRM10, unless you are actually ONLY running an MRM10 (and no other/larger MRMs); or if you have a CT hardpoint and you really want to maximize your MRM capacity.