The Sustain has seven bearings...the Stradic has 5 bearings. The build quality on the Sustain will obviously be better hence the extra cost. Both reels at the 4000 size will have around 20lb of drag. Both waterproof drag systems which is a big bonus.
Both reels would be great to use. I use a 2500 size Stradic which I really enjoy.
Go and pick ALL of these reels up "Shimano and Diawa" and have a good play with them. Really look closely at them. Ask heaps of questions.
I quite often pick up a reel and just think Nope...wouldn't be happy with using that for four hours straight.
All of the reels you've mentioned would be sweet to use and are well built, but you've got to be happy with it at the end of the day.
The next question is how often will you use it. That can be a determining factor when laying a heap of cash on the tackle store counter.
I use my reels once a week so I wouldn't buy a Stella, I'd like one but I can't justify the expense. The Stradic suits my needs for now.
Oh and think about the warranty they offer. They do break !