I betcha there's been a lot of development of the D44 in the last 50 years...
D44 is the rear of choice for the smaller Jeeps, CJ, YJ, XJ, etc. for rock crawling which is REALLY hard on diffs. You guys think drag racing is tough, you ought to see these guys stuck on a rock and literally bouncing the whole rig up and down on 38" tires with 8 lbs of air in them. Talk about shock load...
Not to mention the D44 comes in the back of the Viper and older Vettes. The diff Scottie went 10 secs on was a D44 or a D36, I am pretty sure his was the D44.
You might want to try and compare the two, or at least find out the max potential of both. The thing to do for a solid axle D44 is start looking at off road magazine articles and websites and see what people are doing to beef them up. I wouldn't count the D44 out, although the full floater 9" that Tim suggests is basically a NASCAR rear end and is going to be stronger and maybe even cheaper with the number of circle track guys out there. Check circle track magazines and suppliers for the 9" stuff.