Hi Mike, these products have different hardware and software.
Hardware: the 375GB (and 250GB and 314GB) is a 1-disk, 2-head platform in 7mm high form-factor. The 1TB is 2-disk, 4-head in 9.5mm high form-factor. The 375 has lower demand on the power system. The data throughput performance are similar between the two (both exceed the capability of the Pi's USB2 I/O). The 375 has some additional features to reduce power demand that may increase the spin-up time (power-on to ready) and seek time. These are probably not noticeable in system use, however.
Software: both Foundation Edition (375) and Berryboot (1TB) are multi-boot OS loaders. After installation, when the system is booted a menu is presented that allows the user to select which OS/application to launch. At a high-level they are similar in purpose, but their architectures are very different.
Foundation Edition is based on the R-Pi Foundation's NOOBS software. NOOBS enables download of several OS/apps from the R-Pi site (e.g. Raspbian, OSMC, LibrELEC, etc.) and installs them into separate partitions. NOOBS is designed for new users, providing simple installation of ready-to-run applications. Foundation Edition uses the NOOBS multi-boot system, but is designed for creating multiple software projects (via command-line programming). It enables installation of multiple instances of the core OS (Raspbian Lite), so users can create and run multiple projects from the system instead of managing this on a stack of SD cards. Foundation Edition installs the OSes on the PiDrive versus NOOBS that only supports SD card.
Berryboot also has an online repository with OS/apps for download and installation. There's a wider selection of OSes for Berryboot versus NOOBS. One key difference is that Berryboot uses versions of the OSes that have been converted so they can be installed on a common kernel and the files are compressed for download using a different compression method than NOOBS and Foundation-Edition. Due to the use of converted OSes, updating of the OSes is done by Berryboot's developer at his discretion. Berryboot is designed to provide easy installation of a wide range of popular OS/applications.
Foundation Edition software and Berryboot both work on all PiDrives, so you can try them both whether you have a 375GB or 1TB. The software can be downloaded at the link below.
Hopefully this gives you some idea of the differences. Please ask any questions. Thanks!