I believe all psychotic disorders are on a continuum, or rather, there is one psychotic spectrum disorder. The chance of SZA/BP or SZ is higher the more severe your (underlying) problems are. I believe there is a higher chance of BPD when the underlying problems are more severe, but generally less severe than for just SZA/BP or SZ. The worst BP-I might correlate most with BPD. There is (rather enough) suggestive evidence for all/most of this.

But I believe the true decisive factors in making the disorder more or less SZ, are the opportunities one has to escape one's predicament and how one chooses to escape one's predicament. These factors can more or less be reduced to fear. If one chooses to try and seize an existing opportunity, the fear being moderate, one may get manic, if one chooses not to pursue an opportunity, maybe not seeing/having any opportunity, fear being high, one may look for ways to accept it and to go on.

Both are forms of psychosis, but the one more expressive, the other more impressive. Both reduce fear. The more expressive one causes more severe depressions, the more impressive one has more lasting effects on the mind (and brain).

Both might be best given the circumstances, but I believe a borderline personality can be more helpful still. It's in some ways a combination. Expression can be both healthy and damaging.

The entire thing is a balance between risk-taking and risk-aversion, non-acceptance and acceptance, I'd say.