I haven't tried into this business but, it's best to do good homework as you are doing now, discuss more find more and pros - cons both, rest have a look into these providers give it a search -
and the find their competitors and discuss with all and find all information good or bad after-sales service record as well so to avoid surprises once you will be focusing on your business, leads and clients to make and take the best decisions for all related to you, Wish you all the best.
I can advice you as a loyal customer of business of this kind. I order 2-3 items a month. I order regularly heat print logos and different ornaments on my clothes. So to make it clear I am sharing my experience as a client not as an owner of a business. I have asked the girl from the support team what machines they are working with and I was told that its stahls. The print is nice, it doesn't smell at all and it stays quite long (after washing and drying machine). Hope that you will find this helpful. Best of luck in achieving your goals.