What ZPLPreview gets right
The framing is sharp: preview the label before it goes to a printer or a shared print server, which is exactly the moment a warehouse wants to catch a bad template. Density is adjustable across the full 6 to 24 dpmm range, and a preview link carries both the ZPL and the density with it.
Its conversion list into ZPL is unusually long — HTML, EPL, CPCL and TSPL among them — and its interface is offered in more than twenty languages.
Where the two diverge
Their own site describes the free tools as best suited to single labels, with batch review, approval reports, private templates and API rendering listed under paid services. That is a reasonable way to build a business, and it is the line this tool does not draw: batches are the free case here, not the upsell.
The other difference is where the work happens. We say plainly that rendering is local and nothing is uploaded; their site does not state where conversion runs, so it is not something we can compare on their behalf.
Both give you a link, for different reasons
Shareable preview links are the one feature both tools landed on independently, and for the same reason: the fastest way to settle an argument about a label is to send someone the label.
Here the link is a fragment carrying the ZPL and the print settings, so it opens straight into the workbench with the same density and size — and, because it is a fragment, the label never reaches a server on its way to the person you sent it to.
When ZPLPreview is the right answer
If your input is HTML, EPL, CPCL or TSPL rather than ZPL, their converters cover formats ours does not.
And if you need approval reports or private templates as a team workflow, that is a product we do not offer at any price.