One thing that hasn't been mentioned yet is that the Allvit has a much larger capacity due the linkage. The factory specs for the Svelto topped out at a 22 or 24T large cog, depending on the version, and only had a 26-28T chain wrap capacity. It was more of a competition model, spec'd on bicycles with a relatively narrow gear range. The Allvit could handle up to a 30T large cog and had a 34T chain wrap capacity. For some riders, that 6-8T extra capacity could be a deciding factor.
However, I agree with everything else. The Allvit was heavier and could require regular maintenance to keep the pivots tight. While the integral bash guard could pack the parallelogram with dirt, it also probably saved the derailleur innumerable times on entry level, boom era bicycles. The Allvit had the superior shifting. The Svelto was light by comparison and required less maintenance but did not shift as well. It clearly showed its influence on the later and more famous Jubilee, which is what I consider to be its main claim to fame.