I have used a different vendor for my US t-shirt orders through my Shopify store purely based on price, but they don't integrate with Etsy and I am about to cut loose the higher expense of shopify and move all my operations to Etsy. From my perspective, I am still going to save money with the higher printful pricing and lower cost of using Etsy. I haven't crunched the numbers yet against my other vendor, but it will be overall savings.
The next advantage of using printful is the European local shipping and I can run both US and international orders out of Latvia to my UK and global customers. With the Etsy integration product management is all in one place at my printful dashboard.
In summary... after trying just about everything else, their investments in the framework like integrating with Etsy, European expansion, the quality of the products and fast shipping, great customer service is worth paying a little more for in the end.