When I went into the local JD dealer to pick up my dads 445 I looked at there selection of X tractors, they went up to the X700 series and honestly I wasn't a huge fan of the redesign vs the 400 series late 90’s and early 2000s. The 445 my dad has, has a true liquid cooled VTwin Kal, the motor in the new x500-x700s looks like a B&S motor even know they claim it's a Vtwin Kal still. I believe the 500 series was air cooled also but I remember the 700 series being liquid cooled.
I hate there new hood, I love the 3 piece of the old 400 series, the VTwin in those old 400 series were a solid 2+k hour engines, my dads broke 1000 hours last season and it's been in the shop once, it was in the shop for a hydraulic leak where the deff lock goes in the wet rear, it was a pain in the azz to get to so we said screw it and had the dealer do it, $15 in seals and 2 hours labor.
I currently use a GT husky with 24hp and 54in deck, I picked it up used with 200 hours on it for $800 off CL, that being said it doesn't hold a candle towards my dads 445.
JD makes a fine Garden Tractor, but from what I have seen of there 300 and even lower 500 series tractors, either JD has cheapened there product or Husqvarna has closed the gap, those aren't built much heavier than my GT is.
If I had the money I would be buying a 700 series tractor, I wouldn't gain much of an advantage with anything less, my mowing time wouldn't be improved much, the JD might last longer but it's also way more expensive.
Best bang for your buck would be spending $1500-$3500 on a used 445 with under 1000 hours on it, I didn't see much of a difference in the 700 series from the 445, 4x4 455 would be epic.
My dad paid $8500-$9000 for his 445 in 1999, that's about what the 700 series are bringing today, so price wise they have come down on price if you enter inflation ext.