I prefer Xano for my backend integrations, I know it was not one of the two you listed, but it is a very powerful no-code backend. PragmaFlow (our company) is an Integromat partner, so between Zapier and Integromat I would need to choose Integromat to be a good partner, however, partnership aside I think Integromat is the better option because it allows far more flexibility over Zapier.
I think if you went with Zapier, you would hit a wall relatively quickly depending on what you want to do. Integromat and Xano do not have the wall and 6 months from now you will be happy to have went with either over Zapier.
Agree with @TKOTC. I prefer integromat over zapier it is less pricey and also very powerful.
Furthermore if you decide to use something like xano for external collections, you can do everything there without using integromat/zapier etc, but at the expense of a significant learning curve.