Version 12 introduced a vectorized, scalable UI, allowing it to look sharp on high-resolution displays through the FL64 (scaled).exe option. Legacy Note & Lifetime Updates
Image-Line recommends using the version that matches your 3rd party VST plugins (32-bit plugins in the 32-bit host) to avoid performance issues with "bridging".
The 64-bit version can access up to 512 GB of physical RAM, whereas the 32-bit version is limited to 4 GB.