st. thomas on which is worse: heresy or schism

On the contrary, That which results from an addition to something
else surpasses that thing either in good or in evil. Now heresy results
from something being added to schism, for it adds corrupt doctrine, as
Jerome declares in the passage quoted above (1, ad 3). Therefore schism
is a less grievous sin than unbelief.

I answer that, The gravity of a sin can be considered in two ways:
first, according to the species of that sin, secondly, according to its
circumstances. And since particular circumstances are infinite in
number, so too they can be varied in an infinite number of ways:
wherefore if one were to ask in general which of two sins is the
graver, the question must be understood to refer to the gravity derived
from the sin’s genus. Now the genus or species of a sin is taken from
its object, as shown above (I-II, 72, 1; I-II, 73, 3). Wherefore the
sin which is opposed to the greater good is, in respect of its genus,
more grievous, for instance a sin committed against God is graver than
a sin committed against one’s neighbor.

Now it is evident that unbelief is a sin committed against God
Himself, according as He is Himself the First Truth, on which faith is
founded; whereas schism is opposed to ecclesiastical unity, which is a
participated good, and a lesser good than God Himself. Wherefore it is
manifest that the sin of unbelief is generically more grievous than the
sin of schism, although it may happen that a particular schismatic sins
more grievously than a particular unbeliever, either because his
contempt is greater, or because his sin is a source of greater danger,
or for some similar reason.

Read the whole thing here.  First at T19.