This has been very helpful thanks youit's a very personal choice and you will hear both sides of the argument but at the end of the day it's what you and more importantly your clients think is the best fit for them
I have used both systems and think they both have pros and cons.
CND Shellac is my preferred choice. I love that it requires no buffing, 5/8 min soak off with new top coat, huge brand awareness, great retail opportunities with Vinylux and Solar oil selling itself and also the best training available. I don't believe there is another company out there who go into as much detail as CND and it has certainly helped me be a true nail professional. Clients have most definitely picked up on my knowledge and trust me more.
Some of the cons for me are the old top coat gets thick quite quickly, I much prefer the new top coat. people say they lack colours but honestly I have about 85 colours and I never get asked for anything else. some colour coats can also thicken if not shaken correctly throughout their usage so you must remember to shake shake shake before use
I actually started out with Gelish, some of the pros are you get more services per bottle, it can work slightly better on damaged nails ( not all damaged nails mind , gel polish just isn't that strong and not designed for damages nails) , they do have funky colours that appeal to a younger clientele. it doesn't get thick but can cure in the bottle if you don't cover the window at the front of the bottle
the cons for me were buffing the nails, I think this is now optional or some techs have just opted not to buff but at the time we were told to buff ( this was about 5 years ago now in fairness ). There quality control is not great, there are colours that just aren't photo stable and fade horribly even when not in strong sun, some colours will change according to the batch so even if you order the same colour it may be slightly different, or wildly different as with one colour I remember. I personally found it very difficult to remove but others say they don't find it hard at all , I used to buff the top coat trying to get it to come off quicker and it always took 20-30 mins, very frustrating particularly in a salon environment.
That probably doesn't help much you can also do a search as I'm sure there have been a good few threads that have discussed this topic so you might get more of idea reading those threads too.
Why not try both and see what you like?