I've been doing Print on Demand (POD) for about 3 years. I sell 300 plus shirts a month through the Amazon Merch Program as well as selling on other platform sites like Redbubble, Society 6, Etsy, etc. I also run a niche Shopify store selling POD items.
The advent of Print on Demand sites was awesome in the sense it lowers the barrier of entry to starting a novelty tee shirt company or apparel brand. That's also a negative in that it's brought out a ton of competition.
The biggest hurlde I see with the POD business model is the pricing. Understandably since were not stocking any inventory and really doing no work we shouldn't get to keep 100% of the profits. Obviously the companies offering these services are doing most of the work and are going to charge a hefty fee. Something like Printful for example on a Bella Canva tee after the $5 shipping charge comes to essentially an $18 base price.
In my opinion $20 is about as high as I would pay for most tee shirts and at $20 that only leaves you a $2 profit margin. On sites like Etsy which are geared towards artists people are often willing to pay a bit more to support an artist and for something unique but it is a challenge. When it comes to running a Woo Commerce or Shopify store you can also oftentimes charge a bit more as unlike eBay and Amazon potential customers are not seeing other people's products and prices.
While I see people charging upwards of $50 for a shirt, I highly doubt many of them sell much. I think $24.95 with a $2 or $3 shipping leaves a bit more room for profit and is still fairly reasonable.
In terms of Printify vs Printful, Printful definitely has a better UI and interface and I trust their platform and company more. That said Printify is oftentimes cheaper.
One more POD supplier I'll throw out there who I'm not a big fan of for my business, but whom may work for you is CustomCat. CustomCat is famous for their $6 Gildan 200 shirt which is one of the cheapest if not the cheapest POD shirt available. I was not impressed with the quality of the shirt or printing for my brand, however if you're selling more novelty type shirts and the material isn't as important it leaves a lot more room for profit.
Both are fully capable of providing print-on-demand fulfillment for WooCommerce (as well as Shopify).
Printful is growing quickly and has more product options. They typically have better deliver time, though Printify often has better pricing.
It depends a bit on what types of products you plan to offer. Both allow merchants to sign up for free, so you can see pricing, available products, and product creation methods.
In addition to these two, there are a number of other print-on-demand platforms. Many are alike, but there are some unique differences regarding products offered, shipping policies, etc.
I publish POD Weekly, a blog and newsletter for print-on-demand sellers so I follow the industry closely. [more here: https://merchreadydesigns.com/blog/ ]
Feel free to schedule a call with me if you have any other questions regarding print-on-demand ecommerce!