#1, I would think whomever (Juniper Partner) you would be purchasing these switches through (maybe the same people you purchased your EX3300 from?) would be able to answer all of your questions, quite easily. Anyway,
Yes, EX3400 is the next-gen model of the EX3300. The next-gen models of EX run slight modified version of Junos CLI, referred to as ELS. So if you use CLI, there will be minor differences, mainly around use of IRB vs vlan.0 for L3 IP. There is a new Day One book covering the next-gen switches which notes the differences.
At this time, for new purchases where specific product and SW certification is not a requirement (think SP's), Juniper recommends purchasing the newer model switches, which have following advantages:
1. Can support newer SW code releases. Older (legacy) EX switches are capped at version 15.1 code, with most deployments still running (very stable) 12.3 [R12-S?] code stream.
2. New models are generally priced lower!
3. New models have 40GE/QSFP+ interfaces, which can be used for either VC connections or uplinks.
4. Specifically to EX3400 (vs EX3300), the EX3400 supports dual power supplies. Both come with built-in 10GE uplink ports.
NOTE: The newer models and the older models do NOT VC together! You can not add an EX3400 into an existing EX3300 VC.
In general, EX3400 is most likely the new model of choice for L2 Closet deployments, while EX4300 still remains a viable Closet choice, is more likely to be used in DC for 1GE connections. EX4300 has more 40GE interfaces (6 vs 4) than EX3400. On www.juniper.net under switching poducts, you can find a page that will allow for direct product comparison.
EX4200 is also an older legacy EX switch. Its next-gen replacement is EX4300, which now has 3 different model types - T (no POE), P (POE+) and MP (Multi-Gig capable plus POE++). Again, EX4200 and EX4300 can NOT VC together.
As for license question, there is a change from older legacy EX to next-gen models. The original EX switch provide L3 (outside of static) in the Base; the new models are L2 only in Base. If you want L3, that a license is needed. IGMP-Snooping (L2) is in the BASE! This is a point of confusion for all. To just run L2 IGMP-Snooping on any EX Access product, no extra license is needed. To run L3 PIM, yes an extra license is needed.
Just FYI to all, but is very likely within next 6 months, potentially much sooner, End of Sale (there is always a 6 month last buy window from announcement date) of the legacy EX switches (EX4200/EX3300/EX2200) will be announced.