Mower recommendation

I recently purchased a 6+ acre property and currently mow about 4 acres. I own a 2010 Yard Man 50" zero turn mower with about 400 hours that isn't cutting it. I also mow 5 other smaller yards on the side with a Toro push mower, but want to upgrade to something bigger/better. Assuming I can only buy one mower, what are your recommendations? Or should I really be looking at a new ZTR and walk-behind to mow the side jobs?

If it's only 1 mower, make sure it's not too big for the various side yards. Corners, turns, backing up etc all become a chore the bigger you go with the machine. Having to mow an extra row or two in the open areas won't annoy you nearly as much by comparison.