While the SGT always has a similar GT model, it is not just a longer wheelbase. They had higher hp engines, larger rear and front wheels and tires, of course weigh more. Add it all together and they are more capable machines for heavier use than their GT brothers.
The newer GT decks are so much larger and heavier than the old "white" decks that to me its a bit over the top. We ran a 50" white deck for many years with a ton of hard cutting and it did a great job, not sure my newer yellow decks are any better, and they sure are hard to handle.
Your 3185 compares well with an 1864. I run both an 1864 and a 3204 so I have a good feel for the similarities and differences. The 3000 series is a bit heavier, the decks nearly identical. The engines are the same Kohler Command. The 3000 has shaft drive deck and foot pedals while the 1864 has belt drive and hand control. Thats about it for the differences as far as I know. The 3204 handles heavy grass a bit better than the 1864, not sure if its the extra 2 hp or the shaft drive deck or a bit of both.
The older Cubs were great machines, but so are some of the newer ones. I'm partial to the 1864/2084 series of the '90's so that's my preference right now.