Gelish vs Shellac
Shellac is made by CND (Creative Nail Design). Shellac is more easily found due to CND being a more popular and bigger brand. They have done a fantastic job at launching an advertising campaign which is why people go looking for Shellac.
Gelish, which is made by Harmony, boasts longer wear, bigger bottles and has lots more colours - 80 at least - with exciting sparkly and neon colours. The colours in the Gelish range are a stronger hue than Shellac. Shellac only has 40 or so colours. Gelish tends to be less expensive too as the bottles are less expensive. Gelish is also a true gel polish, a mixture of gel and pigment, whereas Shellac is gel and polish. This makes Gelish slightly thicker, which means it lasts longer. Harmony only sell Gelish to Salons so as avoid knock offs and fakes from diluting their market, which means that clients feel that they are getting a superior product.
One thing I would say is that the hybrid gel polishes such as Shellac seem to be more suited to nails that are strong, as those with weak flaky peeling nails would be better suited to a stronger gel like Gelish or better still Bio Sculpture. But both these gels do incur less damage to the nail plate than Bio sculpture gel.
Both makes otherwise are applied like a polish, are cured with UV (or LED lamps which makes curing time even quicker with Gelish) and both makes last longer than normal polish. Both can layer colours and nail art is easy. Neither needs the natural nail buffed before application, Shellac's top coat is reported not to be as shiny as the Gelish super long lasting mirror shine without the need to reapply the top coat but both soak off easily. Both are compatible systems with each other and also with Bio Sculpture gel. But as I say, companies tend to be jealous of Gelish as it has more going for it.
Gelish also has a structure gel which gives additional support to the nail, so ski jump nails for example can be corrected in the same way that Bio sculpture gel offers sculpting gel. However, Gelish structure gel doesn’t have the same strength for extensions as is possible with sculpting gel.
As a last word, don’t just take my word for it that Gelish is better. You will find that other nail techs and clients prefer Gelish over Shellac for the same reasons that I do.