Unless there is significant tangible assets involved, I would always go for asset purchase that will shield you from all liabilities that will come with the stock purchase of the subsidiary that they the seller would first need to spin-off into a new Co. Make sure it come with all the IP related to the product you're interested in

The preference is usually an asset purchase for reasons already stated in this thread. But, there's compelling reasons that support either approach - stock or asset purchase. Really need to dig in on what you need to buy from the Seller aside from the SaaS product itself, ie, MSA's, key employees / developers, whether key contracts can be transferred, whether existing customers buy only the SaaS product or do they buy other services/products from the Seller, etc.