Well, the Tessar is an Anastigmat here one of the groups had aquired an airspace, and the Heliar is a triplet where the outer elements are split into two cemented elements...
And that was the simple answer.
Added to that, the Heliar was designed with some residual spherical aberration, which serves to smooth out the transition from sharp in-focus to the blurry out-of-focus areas. Or at least that's what Voigtländer said in the 1930's.
Incidentally, the Heliar you have is most likely to be a Dynar: Same thing, only the two elements in the outer groups have swapped places. Voigtländer liked the Dynar lens best, but preferred the Heliar name.
The Heliar seems to have a little more image circle, at least from my test of lots of 135-150mm lenses on 5x7" film.
I like my Heliars, but not enough to take them off the Bergheils they came on (6.5x9 w/120mm Heliar, 9x12 w/150mm Heliar).