If Marcionism could be a "third way of Christianity," then
Montanism might well have been a fourth, some of the Christological heresies (such as
Arianism) might have been a fifth. The medieval
Waldenses, a sixth. And on it goes.
Most people forget that there has never, ever been, in the entire history of Christianity, a
single Christianity. There has always been
varied belief abouth Jesus. There has NEVER been a shortage of "ways" of Christianity.
As for whether Marcionism was anti-Semitic ... that is a difficult claim to evaluate, since dislike of Jews was common in his time. (As was the case for most minority ethnic groups ... Greco-Romans were not known for being all that tolerant of strangers!) We would have to come up with some sort of "Jewish antipathy" metric, then compare how Marcionism measures on that scale, to the antipathy of others. Offhand I'd say this would be a very iffy venture at best, and impossible at worst.
What we do know about Marcion himself is that he had a very poor opinion of
anyone who did not share his beliefs; he does not appear to have held Jews in much more contempt than he held Christians outside of his own church. It is very likely that his followers may have used his teachings against Jews, yes ... but then, among
other Christian communities, other Christian teachers had also written anti-Jewish polemics, and
those writings were also used by
their followers against Jews.
A good example is
Tertullian. He opposed Marcionism as vehemently as anyone else, but simultaneously he also vilified Jews. Terullian's
Answer to the Jews, for instance, shows that non-Marcion Christians despised Jews as well, on different grounds than Marcion.
So were Marcion's teachings harsher on Jews than those of other sorts of Christians? Given the historical record, I'd say not.