I have worked as an IFT paramedic for the past five years. I can honestly say that I've learned more doing IFT than I ever did 911 in the same amount of time. I work for a service that does critical care transport so I am called to manage some really sick patients (i.e. multiple vasopressors, vented, etc). If you really want to be a healthcare provider then IFT is where its at. I have done more pharmacologically speaking than you ever will as a 911 provider. As for psychomotor skills, I definitely get my fair share of IV insertions. There is nothing like getting a vented patient with a single 20g peripheral IV so I pop in a second line quite often. And we get called to the Urgent Care my hospital owns to take patients to the ED so they get the full workup with IV, meds, 12-lead, blood draw, etc. And we get called to nursing homes for everything from sepsis to chest pain to abd pain. I've done EJs working IFT. Intubation skills take a hit because our patients are already intubated but if you work 911 part time somewhere you can make up for that.
The benefit of working IFT is knowing your patients diagnosis and being able to correlate it to what you see in front of you. You will learn labs like the back of your hand. IFT is a great environment for learning to be a clinician. I would never say your knowledge base takes a hit. In fact, I would say the opposite. You will grow your knowledge base tremendously especially if you're working for a company that does critical care transport and doesn't rely on a nurse.
With all of that said though I definitely recommend getting some solid 911 experience. 911 and IFT are completely different worlds. I feel that working as an IFT provider has made me a better pre-hospital provider. This has been my experience, others may be drastically different.