Both are good guns, which is better for you depends upon how you want to use it.
The Tracker is smaller and lighter. From what I can gather researching online, the Tracker isn't 28oz as listed on the website (that was the weight of the long ago discontinued Titanium Tracker), but more like 34oz. For a carry gun, the Tracker's weight is a bit higher than most people like, but wouldn't be bad. The 66 at 38oz may be over most people's weight limit for carry (but with a good holster and belt it can certainly be done). I would suspect that the 66 is more durable with extended .357mag use than the Tracker, but I have not heard of any issues with the Tracker.
For me, the biggest deciding factor that will lead me to more likely pick up the 66 over the Tracker is the porting. Even if I would use it as a carry gun, I'd go with the 66 despite the extra weight. Porting will push the flash up right into your line of vision and wreak havoc with your night vision, which would be a huge problem in the typical low-light self-defense scenario. Also, the ports will make the already loud .357mag even louder and likely cause hearing damage if used indoors. So, for me, I'd only get the Tracker as a fun gun at the range and maybe as a woods gun (though the 66 would be fine as a woods gun as well), and I'd stick with the 66 if you are thinking of using it as a carry or home defense gun.
A less important issue overall, but one that will matter for many people, is the availability of grips. With the 66 you will have far more options if the factory grips are not what you want. You may also find the 66 easier to find a holster for than the Tracker (more holster makers make holsters specifically for the 66 than for the Tracker, though many K-frame and L-frame holsters should fit both guns).