Which social media platform would you go for marketing your sass products.

Depends on who the ICP for the Product is. If you have a very visual product & arguably more B2C focused, Instagram is great. If however you're more B2B, then LinkedIn/Twitter can be better.

you should choose the one where your audience is In addition to Instagram and twitter, you also have facebook, Linkedin, and many others

In our case, Twitter is working very well and we were able to build a strong community around our product. We also use Reddit to bring more users :)