I use both Fabri-Tac & Magna-Tac routinely. I personally can't tell the difference looking at them and the bottles don't give many clues (like ingredients). Some of my Fabri-Tac lined clips have held for a while but much later come loose, while none of my Magna-Tac lined clips have come loose, but there might be other factors to consider. The majority of my Fabri-Tac lined clips have held.
The Adhesive Selection chart on Beacon Adhesive's website lists Fabri-Tac as best for beads/gems, canvas, cardboard, fabric, leather, papers, photos, and wood. Magna-Tac is listed as best for beads/gems, canvas, cardboard, ceramics, fabrics, glass, leather, metal, and stone. But, elsewhere on the site, it says Fabri-Tac bonds glass and Magna-Tac bonds wood (and it says Magna-Tac bonds wood right on the bottle!). You would think that they would have the same indicators on the chart if they were the same product. But you would also think the company president would know what he is talking about