Hi,
EX3400 has low end ARM processor when compared to EX4300. However EX3400 can be used for normal Layer 2 Swithching purpose. EX3400 offers
1) Virtual chassis and
2) Can act as Satellite device for Junos Fusion with EX9200 as Aggregated device.
EX4300 device provides more options like,
1) Virtual chassis with Mixed platform i.e. with QFX5100.
2) It can be part of Virtual chassis Fabric as Leaf device.
3) Act as Satellite device in Junos Fusion environment.