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, . . .
I agree with rccpgm except that QFX5200-32C can actually do EVPN-VXLAN - but only for layer2. VXLAN routing is not supported on the Tomahawk chipset.
I would still go for the QFX5120-32C as it has a way better chipset for a leaf device and has just recently been introduced with many years before reaching EOS/EOL. QFX5200-32C is probably closer to EOS but Juniper has a general tendency of shipping platforms for many years. The EX3300 had a 8 year lifetime before being announced EOL - 8 years is quite long in the IT industry :-)
On top in the favor of QFX5120-32C... the port-to-port latency between 25G ports on Tomahawk based chipsets are horrible compared to Trident based ones. So if you want to do 25G initially the don't look at QFX5200-32C.
But I would also recommend that you discuss your design with your local Juniper partner to ensure the design matches your expectations - just as rccpgm mentions :-)
-- Best regards,
Jonas Hauge Klingenberg Systems Engineer, SEC DATACOM A/S (Denmark)