Forgive me, but this will be a bit longwinded....
Addressing your questions first:
#1- "Weight - I know heavier is better in tractors, but what about riding mowers on slopes?"
Heavier CAN be better when it comes to lawn mowers, but not always. Unless you have access to a host of information I don't know how to obtain... CG (center of gravity), tire contact patch, track width, etc... there's no way to specifically determine the true slope stability of a mower. That being said, I've found that a little common sense eyeballing of the track width and tire size to be a better guide. Turf tires can't take advantage of the added weight the way a tractor's ag pattern tires can... and a heavier mower may not necessarily have a larger tire or contact patch than a comparable lighter mower. With mowers and turf tires, absolute TRACTION is mostly achieved with contact patch, not weight. On the flip side, wheel weights and high density fluid filled tires are sold every day for a reason... however, it's not the added weight that helps so much as where it's placed. Wheel weights and fluid filled tires move the center of gravity lower, which inherently increases stability on slopes. This is why I say heavier isn't always better. If the heavier mower's weight comes from that deluxe seat, engine, heavier hood/fenders, or anything above the centerline of the drive axle... it's hurting, not helping. I will say this: a heavier and wider deck can make a very noticeable difference in stability on slopes, as it is the lowest weighted part of the machine. There are exceptions to this as well... but I don't think anything in your price range will have a true full-float deck with hydraulic drive, unless you buy a used commercial unit.
#2 "Durability: The Husky has aluminum spindles - are they problems? Their 54" model has cast iron."
Aluminum spindles are designed for mowers that mow flat and well conditioned lawns. I would say that's a no-go. When slopes are a concern, you should always go with a cast iron front assembly, if but for nothing more than the aforementioned center of gravity. You can rest assured that the aluminum spindles are engineered to serve their purpose, especially in a Husky product... but if you read the owner's manual, you'll likely find that your demands will exceed their specifications. Also, the "my wife seems to BREAK something on the L120 EVERY TIME SHE USES IT" should be enough to answer that question. However, if that lowly L120 has lasted you 10yrs, the uprated Husky will likely have a comparable service life.
Now that I've answered your questions, I have a couple.
#1- why "gasoline only"?
#2- why 2wd with a locking diff, presumably ruling out a 4wd unit?
#3- have you ever tried a front mount mower?
#4- how many acres will you be maintaining with this mower?
#5- how long does it currently take you to mow?
#5- are you absolutely set on buying new?... why? (?warranty?)
I have a couple suggestions, but I'd rather fill in some gaps first.
