Gloss sealer is inherently stronger than matt sealer. Just is, presumably a consequence of how the matt effect is produced.
So the top of line approach is to put down a coat or two of gloss sealer, then a coat of matt sealer. When you notice that the mini's showing shiny patches where the matt's worn off, then it's time for a light recoat of matt. Highly recommended for minis that are going to get a lot of handling.
As for brands? I recommend Humbrol Acrylic Matt Sealer. I did use Dullcote last year, when we couldn't get Humbrol in Australia, and I found it far more demanding. You had to shake it for three to five minutes, otherwise it gave a semi-gloss effect. The Humbrol only needs 30-45 seconds of shaking to give a reliably flat finish.
Both repay the standard sealer conditions - not too cold, and not too humid. You can stick a can for a few minutes to warm it up - the limitation is on the contents temp, not the drying temp - but high humidity will screw you over.