if I correctly understand you question, your concern about the connectivity from the edge FWs to the network Core if its MEC (vPC/VSS) vs. Routed L3
with the MEC vPC and VSS they work a slightly deferent as with vPC it will be a bit tricky to have routed traffic over the vPC peer link, I think a new NXOS may support it ( yu need to check) you you will need a separate Inter-switch routed interface for the routed FW traffic
with VSS you should be fine
that being said which one better MEC or routed L3, I would look into the convergence time as well as if you need the traffic to be distributed across both Core switches or not. these should be your evaluation criteria.
not sure if you do 0.0.0.0/0 to the core, because I am assuming your FWs connected to the ISP in which the core need to have the default pointing to the FW, unless I am mistaken