Any machine can be overworked, don't get too worried over one instance. As long as you treat ANY machine right, it will treat you right.
The 4105 is a physically larger machine than the 3320. Unless you need the extra HP for the PTO, otherwise per the specs, the two machines are very similar.
I love the plastic. As surprising as that may sound, it is extremely durable. It's a trade off, because I can take a baseball bat to the heavy duty plastic and it won't leave any sign of damage. Do the same thing on steel and you have a bunch of dents. Now, should you really wack something hard, and I mean REALLY hard, then the entire plastic piece is broke, but there's just a large dent in the steel. Either way, for it to look new, the entire piece will have to be replaced.
The 'goodies' of the 3320 are great! There's something to be said about simple. Trust me, I love driving my 1953 Farmall Cub around. There's nothing too the tractor. Three forward speeds, one reverse. No hydraulics whatsoever. That's fun for a little while, but if I had to work with something like that all day I'd be beat! All the comfort tractors are so much nicer than the old. Not saying that the 4105 is comparable to the Cub, it's not, but when you have a choice to run a 3320 or a 4105 all day, the 3320 would win out every time for me. May be a little smaller physically, and a little less HP. But the specs are relatively even and the 3320 is MUCH more comfortable. There's really no need to worry about the extra gizmos in terms of failure either, they are built to last.
As far as lift goes, there's funny things like that. The specs aren't perfect. In real world performance, I doubt you could tell the difference between the two.
I'd go talk to the dealers. Some of those things on the 4105 are optional, where they are standard on the 3320. Have them explain to you and show you the literature on where it says that.
My vote stays for the 3320, I think the one you saw was just a fluke.