I have a 505 I and recommend it for what you want. I doubt if there is much difference in the horsepower needed. I had a 504 F which was easy to pull, but the 505 is not hard to pull.
Advantages of the 504 are smaller bale which can be packed on a trailer without the load being too wide, and there being a little more sale for them since they can be placed in the back of a pickup. Since you don't want to sell hay this shouldn't be a problem, anyway the 505 bale can be put in the back of a pickup. The 504 supposedly makes a neater bale. but I doubt if the cows notice.