I find geleration lasts a lot longer than shellac but shellac comes off easier. Both ranges have great colours and both come from good companies.
I use geleration and love it but the only way you can tell is to go and get both done and see what you think
I love shellac. Not used gelleration but had it done in my own nails. I found the gel spiking was quite uncomfortable and it was a lot harder to remove than shellac. Plus you don't have to buff the natural nail before a shellac application. HOWEVER, with that in mind not everyone gets 2/3 weeks with shellac as its a hybrid power polish rather than shellac. A few of my hairdresser friends need something with more staying power.