They’re both exceptional machines, the 319 is a model up and has the piano keys which provide masses of stitches by combination and also takes cams, the 306 just takes cams. Some people say the 306 is a slightly stronger stitcher and easier to maintain because it doesn’t have all the cams built in, but there doesn’t seem to be a consensus on that. Others love all the beautiful patterns and not inconsiderable strength of the 319.
I haven’t owned either but I’m a bit of a fanboy, and did a fair bit of reading on them.
From the 401 and 500 which have similar built in cams the number of stitches is amazing especially if you go off piste but that’s not to say they’re all useful.