When you write as a "leaf" and MX is your "Spine", are you talking EVPN/VXLAN design or something else? If indeed EVPN/VXLAN spine-leaf design, that QFX5120-32C is probably better suited for leaf role as it can perform VXLAN L3. The QFX5200-32C can not perform VXLAN functions, but is well positioned to be a lean-spine device (IP Fabric).
QFX51xx is Trident based, while QFX52xx is Tomahawk based, and Tomahawk has no VXLAN capabilities, at least not today.
I would think this question would be best discussed with your Partner-Juniper local sales team, not here, . . .