I have no direct experience with these mowers (I run commercial units) but doing a side-by-side comparison of the specs I would definitely recommend the SD for the many upgrades that make it a more robust machine, and if you're going to get the SD you may as well get the 54 inch for the small price difference. In your case I can't see any advantage to getting the 48 and you'd probably regret not getting the larger machine. You likely won't get your lawn cut in an hour at first, but with some experience on the machine it's certainly within its capabilities.