Pay attention to the features supported by all these three platforms.
EX4300 being a newer platform, doesn't support all the features that EX4200 support.
You this link to find out what is supported on all EX platforms:
http://www.juniper.net/techpubs/en_US/release-independent/junos/topics/concept/ex-series-software-features-overview.html
One other thing is the CPU performance, EX4300 being the most powerfull among all of them, followed by EX4200 with EX3300 being the last one.
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