Greetings, you are seeing this difference because the SRX300 uses the enhanced layer 2 software and you SRX240 does not
ELS devices use the "IRB interface" as the SVI, here is a list of devices that are ELS as well, any other device that is not in this list will be considered as a legacy device hence they will use the "vlan interface" as their SVI
If this solves your problem, please mark this post as "Accepted Solution" so we can help others too \:)/
Lil Dexx JNCIE-ENT#863, 3X JNCIP-[SP-ENT-DC], 4X JNCIA [cloud-DevOps-Junos-Design], Champions Ingenius, SSYB
All the legacy SRX series devices with the Junos code 12.3X48 and below uses VLAN interface while the Next-Generation SRX series devices with Junos code 15.1X49 and above uses IRB interfaces. However, for QFX/EX/MX IRB interfaces was introduced way before these Junos versions.
The reason for introducing this IRB interfaces replacing the VLAN is given in the following link - https://networkengineering.stackexchange.com/questions/58449/whats-irb-meaning-and-whats-the-difference-between-it-and-vlan
Excerpt from the above link:
In 2013, Juniper introduced a new configuration syntax for EX switches (starting with Junos 13.2) to remedy this: Enhanced Layer-2 Software (ELS). This syntax is similar to the syntax used on MX routers. One of the changes in the new syntax is renaming vlan interfaces from vlan.xx to irb.xx. You can find a complete list of all changes on the juniper.net website.