In summary they are more expensive and lack technical resources. In our experience they were very responsive when they were trying to make us their customers, but after that it was really hard to reach out to them.
Regarding TrueCommerce, there is a pre-built connection between SPS Commerce and TrueCommerce, so technically you can easily have access to all SPS partners through that. I am going live soon with a customer that uses SPS Commerce, while we use TrueCommerce.
The only thing is that SPS started to push a service agreement on whoever is trying to integrate with their customers which for me doesn't make sense at all. I think it's an audacious way of trying to expand and acquiring new customers. In our case we pushed back on all their attempts.
It was a red flag for me as well because, I don't know how, SPS Commerce convinced some of our customers to push all their vendors to be integrated via EDI, and even that our customers would only enter orders to vendors that were integrated via EDI. How come? These projects can take time. How SPS Commerce can handle so many projects at once? It isn't possible in my opinion.