I use both a soak off and a file off gel, and feel that they aren't 2 systems to be compared against one another...each has its own place and each is suited to a particular kind of client.
The soak off gel I use comes in a huge variety of colour and I know I have clients who would have stopped coming along time ago if they didn't have the choice of changing each time, yet they do not want polish. It is probably more suited to an NNO, but IMO length isn't relevant, some of my soak off clients have the most beautiful long natural nails and just love the gels to enhance and strengthen. I do extend nails with my soak off gel and very successfully, but not if a clietn is ahard on her nails.
Soak off is also great for those clients who openly admit they want gels for a special occasion and won't keep them up. In my 'early' days I KNOW I used to put people off coming back to me, as I advocated they should keep their nails up all the time, and never stop.....and some people just simply aren't ever going to do that, and then fel bad coming back as they didn't keep them up. I now offer the service of gels for the 3 or 4 weeks that they last and incorporate a removal and manicure....and I have loads of clients that are more comfortable with that....for example for holidays or weddings. They come back time and time again...and tell all tbeir friends....and everyone is happy!
But for those regular clients who are hard on their nails and need length adding, or real tough strength building in, especially if they have permanent french every time....I always use a file off gel! There is no need to remove their nails, and they keep looking long and strong. I also use a file off gel if a new client walks through the door, plonks 10 chewed fingers down and ants them all extending. I explain we need a little more strength and thats my product of choice for it.
Interestingly I have far less lifting and breakages with my soak off gel, but that may well be down to the nature of the clients too.
So, both work for me!