The 500's have some improvements over the 300's but I don't remember exactly what thet were. If you are going to cut soybeans in rows narrower than 30 in the the 500 is the way to go. I had 2 300 series heads and in narrow row beans they knocked out a lot with the wide snouts. company rep told me once that the 300's were designed for 30 in rows.
300 will work fine in tall beans but the 500 will work better in most conditions. Dead area between sickle and auger was reduced by 3 or 4" on the 500. Good luck
I've heard the 300 won't cut lower than 3", I've been told the 500 will cut lower than 1 1_2". Check your pod set, do you want to leave that much uncutIJ Perhaps 500 users will let us know how much their's leaves.