Will it be complicated to put in place?
No if you know some basic programming. Any FMB family device (excluding FMB64/FM64) - use such codecs for data sending:
[Codec 8] (for records sending, 1B per avl_id)
[Codec 8 Extended] (for records sending, 2B per avl_id)
[Codec 12] (for commands communication FM <-> server)
[Codec 13] (for sending buffered RS232/RS485 packets when captured by modes: tcp ascii buffered, tcp binary bufered)
Are there differences between data sent by the two devices ?
Answer: How sent - no. What is/can be sent - yes.
[https://wiki.teltonika.lt/view/FMB_AVL_ID] This is the general information for all avl_id (IO ID's) -> for various different devices.
From this you can notice - that FMB120 / TMT250 - have many overlapping (the same IO's) - but some are different and specific only for FMB120 for example:
For example Ignition IO -> FMB120 has it, but TMT250 - doesnt.
And vice versa:
116 Charger Connected
310 Movement Event
These IO elements - are exclusive for TMT250/GH52 (handheld devices) and other FMB family devices do not have these IO's.
Small note: there is error on [https://wiki.teltonika.lt/view/FMB_AVL_ID] -> From certain base FW version (03.15.00) -> battery level % IO -> is global for all FMB familty devices which have battery and is no longer exclusive for TMT250/GH52 (handheld) devices only.
That's in general how this goes.