Thanks for raising this question. There are a few answers that I'd like to share:
* splathead was exactly right that depending on the design, you should consider different print techniques. The awesome piece about flex/flock is that you can give the customer more flexibility (allowing them to choose their colors) and offer more outstanding options (like silver/gold, glitter, glow in the dark). Digital Direct (or commonly called
DTG) doesn't offer the color flexibility, but is full color and is an ink versus the vinyl sheets, which leads to a non-plasticky feel which can come from too much flex/flock.
** Note you can improve on this by puzzling rather than layering in your designs. Meaning that there aren't layers of flex over each other, but that the design is puzzled.
* Quality is hard to determine as it depends on the customer. Overall, I can tell you that flex/flock has 2x the customer satisfaction that digital direct does. Both are high in terms of great customer feedback to be clear, but flex/flock rocks the customer's world overall. IMHO this is because it is so striking. It stands out. These customers scores are judged equally as they are both based on our process and service, and it has been consistently like this for the three years I've been with Spreadshirt.
* Regarding availability, Digital Direct is available for more products in North America (US and Canada) than in Europe right now, but we are improving that. We've recently turned on three of the most popular women's products in the EU and will continue to turn on the rest of our apparel selection, as we have in North America. We started with Digital Direct in North America, so that's why there is more available there, but you will see this changing.
Hope that helps!
Flex-print very plastic-like.... 
Re: Flex-print very plastic-like.... 
- I know what you are saying about the flex feeling a little plastic like if there is a ton of it on the shirt.
