Tank tracks

I've used both, and I'd have to go with ModelKasten. Much easier to use. The Fruilmodel tracks I've used all needed a tremendous amount of cleaning and drilling to get them into usable shape. They look great once finished, of course, but only after quite a bit of tedious labor. I also think the Fruilmodel tracks I've used are bit out of scale (too big).
As far as sag goes, the ModelKasten tracks sag beautifully. You don't need the actual weight of metal to do that!
The ModelKasten tracks are superbly molded and dead-on accurate, both in detail and scale. Clean-up is quick and easy, and assembly is a breeze.
Plus, the ModelKasten tracks are much less expensive, at least here in Japan.
Anyway, given a choice, I'd go with ModelKasten every time.
By the way, I DO NOT work for ModelKasten!
This post was starting to sound a bit like an advertisement!

fruil modesimmo I believe are the best. I have never used model kasten tracks though. If you are looking for heavy (these tracks are made of lead) sags in german and russian tracks. fruil modesimo is the way to go. I recommend RJ Products. YOu can find them on any search engine.

Several companies including Fruil Model and Model Kasten have them.
Use the cross reference or database at this site to see most that are available.

http://www.greatmodels.com/