I have not used both so I cannot comment on that specifically. I have heard good things about ProForm as well, so I assume their gear is good quality. I like Stallion becuase my source is top notch and every Stallion product that I have sent to the lab to be tested came out great!
Never tried Stallion. Have read nothing but positives except the loose tops which have been corrected.
ProForm is good affordable gear. The customer service is about as good as you can ask for. The lab gets a big ass A+ from me. I don't think you could go wrong with Stallion or PF. You would save a little cash with PF, I do know that.