From lens edge thickness there is very little difference between
Zenni's 1.57 index as compared to a 1.61 high index lens. You would
have to have a Rx in excess of -8.00 to see a half a millimeter
difference.
The only thing with Zenni is that your frame height must be 32mm or
higher to be able to produce progressives while other known brand name
lens like Rodenstock can produce progressives as low in frame height
as 18mm with a 1.54 mid index lens. So, whether they says high index
or not does not really give you the whole story.
If you provide your Rx here, I can calculate the lens edge thickness
based on the frame dimensions(width x height x nose bridge length) you
choose to see if it worthwhile to pay the extra for a higher index
lens.