Hmm, interesting question. I was wondering this myself the other day. I've used protective styles throughout my whole HHJ so haven't experimented with both possibilities.
I've worn protective styles my whole life (before I knew of the term 'protective styling') and it was only until 2008 when I changed my ENTIRE approach to haircare that my hair got past shoulder length.
As a child I always wore cornrows, as a teenager I always wore braids and phony ponies and in my early 20's it was weaves and those darn phony ponies again.
I think all that time, my hair was still subjected to too much manipulation and abuse hence no retention. I still combed my hair with any type of comb I had to hand everyday, brushed my hair to slick down my edges lol... everyday, and generally didn't handle my hair with care at all. I hardly ever moisturised and used gel
.
Considering that I spent many years at shoulder length, even though I was continuously wearing protective styles, I think a range of things helped me retain length and wearing wigs helped a lot along the way. Also, with wigs the only time I touch my hair is on wash days and each night/morning to moisturise and seal whereas with ponytails I combed and brushed everyday.
OK, so what I'm saying in a long winded way is that the combination of things I've done have helped me grow and retain some length and I think my chosen method of protective styling had allowed me to retain even more length.