Having been in your shoes and having folks tell me the same, it seems as though you are psychologically leaning to the X500 series but have not yet reconciled it in your mind. My advice with these machines is that they do last a long time with good service, which means two things: first, if you buy something you really do not want you will be stuck with it a long time unless you trade, which is always an expensive proposition, and second; these machines over several years offer very little difference in price per year, so a difference of $2-3 thousand dollars is not that great when spread over time and use. Looking at your pix, either a larger 300 series or the 500 series would do fine, but if your question was if you could justify a 500 for your yard, the answer would be yes. If I had your lawn and property I would be getting a 540 and not looking back, though there are some negligible differences in some areas to the higher numbered 300's which I have noted in previous replies.
John M