Myers likewise distinguished between people with mild and strong preferences, and allowed for the possibility of middleness on all four MBTI dimensions.
Nobody knows for sure at this point, but as I understand it, the existing studies suggest that it's likely that most or all of the MBTI dimensions — like the four Big Five dimensions they basically correspond with — exhibit something like a normal distribution, with substantially more people near (or in) the middle than near the extremes. Myers believed that it might turn out that one or more of the dichotomies was truly bimodal to one degree or another — with, in effect, a more or less empty (if narrow) zone in the exact middle of the spectrum. But she never asserted that that theoretical possibility had been factually established by any respectable body of evidence, and the 1985 MBTI Manual (which she co-authored) stressed that the evidence for bimodality was sketchy at best. And since then, as I've said, quite a lot of evidence has accumulated that seems to suggest that most or all of the MBTI dimensions exhibit something more like a normal distribution.
In at least one of the early versions of the MBTI, it was possible to get an "x" on any dimension. The current version assigns people a (tentative) type on each dimension, but that's a very different thing from saying that it isn't possible for someone not to have a preference — and the MBTI Manual specifically notes that someone with a score near the middle is someone who has essentially "split the vote" rather than offered much evidence of a preference.
The "Step II" version of the MBTI includes five "facets" for each dimension — just as the NEO-PI-R has six facets for each Big Five dimension — and allows for the possibility of being, for example, on the S side of three of the facets and the N side of the other two.
So in trying to figure out what label captures you best, I'd say it makes sense to not rule out the possibility that, whether or not it's possible to be in the exact middle on S/N, it may be that a not-insubstantial number of people may have S/N preferences that are sufficiently mild that "x" arguably captures them better than either S or N would.
If you're interested in an "introduction to S & N" I put together a while back (with quotes from Myers and Keirsey), you'll find it in the first spoiler in this post.
And in case they're useful to you, I've put roundups of INTJ and ISTJ profiles in the spoiler below.