I have a DVD with both the 30s version and the 40s version on it. After watching the 30s version, the 40s version is like My Fair Lady or something. It's not bad or anything, but the 30s version really crackles.
It's the difference between pre-Code and post-Code Hollywood. The 30's version is surprising frank and brutal, especially in its depiction of the abusive relationship between Hyde and Ivy, while the 40's version seems rather too tame and tasteful by comparison.
Or so I recall. I haven't seen the Tracy version since I was a kid.
There's an old story about how some celebrated author (Somerset Maugham?) visited the set of the 40s version--and couldn't tell whether Tracy was playing Jekyll and Hyde that day!