One of the benefits of the new Firmware and Cura LE is that TAZ 5’s are no longer required to pre-heat their printer before starting a job. With the firmware and profiles included in Cura LE, the printer will automatically set the bed and hot end temperature, and wait to begin the print until the temp is reached.
The marlin firmware is an open source project, and is constantly being worked and improved upon. The version in Cura Legacy is of the 1.0.0 branch, while the newest stable version released is the 1.1.9 branch. (We are currently testing on as well) You can see a list of all the improvements made here: https://github.com/MarlinFirmware/Marlin/releases We recommend using the version of marlin included within the Cura package, as there are some slicing engine changes that require the newer version.
That being said, If you would like to use the old firmware, you can go to Preferences > Configure Cura > Allow connections to wrong machines. This will allow you to connect to your printer using the older 1.0.0 branch of marlin.