So, I have used both Shellac and Gellish since the beginning! i don't actually care who was the first to market! This is a global economy and the various brands will have different 'launches' in different countries. I'm sure the R&D for both categories have been extensive and probably parallel as the market need was recognised.
As to the difference....there are big differences. They are not necessarily obvious to the client/consumer but I believe the differences are crucial for the professional to understand!
CND have now called Shellac a 'power polish'. It was originally called a 'hybrid' but the consumer didn't understand this term. For professionals it should have been the perfect description. It IS a hybrid. It has the traditional solvent technology of traditional polishes in a clever formulation that harnesses the photoinitiators of UV gels.
It applies to an 'appropriate' natural nail just like a polish and looks like a polish. Under the right circumstances, it will remain perfect and glossy due to the UV cured characteristics but will release from the nail with ease due to its traditional polish characteristics. (I use the word 'appropriate' as not every nail is right just like not every nail will hold onto a polish without chipping. This also applies to gel polishes!)
Shellac is a traditional polish with solvents that harnesses UV technology to overcome evaporation as a method of drying/curing. The 'tiny tunnels' analogy shows how a solvent will permeate the coating and release it, in flakes, from the nail. This is quite specific to Shellac.
Shellac has not specific strength for the natural nail that is any more than traditional polish. A weak nail will not benefit that much more than polish but it's longevity can help, in general, protect nails.
Benefits of Shellac is that there is (usually) no need to buff the nail plate before application (which over many applications that require buffing will thin the nail plate). It is applied just like polish and it looks just like polish. It will release quickly without having to buff the surface of the coating. The downsides for Shellac is that the strength it gives to the natural nail is not that much more than traditional polish.
Gel polishes only use the UV curing technology. There are no solvents in gel polishes. Some brands use the 'no solvents' as a marketing plus but I don't believe this is relevant!
They are, in simplistic form, a low viscosity coloured gel. This has the capacity to provide some extra strength to a nail but has a slightly different appearance from a traditional polish.
Most gel polishes recommend a very light buff on the nail plate before application. This is not always necessary as some nail plate will hold on to the product without this. The 'tiny tunnels' don't apply to gel polishes and this can be seen as the coating usually comes away in a bigger piece. The soaking off takes marginally longer as the solvent needs to get through to the base coat to make it 'release' from the nail plate.
From a technological point of view, they are different. One is very close to a traditional polish with no 'health' issue to the natural nail. The other has a minimal 'health' issue but, in the hands of a good technician, this is negligible.
Removal is key to both! The most damage is caused by a rough removal. But this is the subject of a different thread.
Then there is the marketing aspect! Brand awareness is absolutely key to the success for the technician! Clients are hugely influenced by the media profile of the brand! It doesn't matter if you agree with this, it is a fact! Clients will ask for a brand although they have no idea what they are asking for!
Following this thread, people ask for 'Shellac'. This is because the marketing is working! Gellish is doing a fantastic job with the 'celebrity' aspect. In general, the consumer don't understand any differences They ask for what they have heard!
EVERY technician needs to understand the technology of EVERY product they use. This is what makes a professional different from a 'hobbyist' and an amateur!