Well, in my opinion I think that you are both right. Only real way to see what works best is to build a quick prototype. However, if you cant do that, then building it to certain ergonomic sizes will help make the seat 'fit' a general customer.
Here is some basic information for you:
The seat height should be shorter than the popliteal height of the user. Being that this is not a task chair, the tolerances are slightly larger than the 1.5" recommended. However, we have all experienced the feeling of a chair that is too low and our knees are bent way too much (uncomfortable). Also, the seat depth is a major concern, as you dont want your popliteal area (behind the knee) hitting the chair. This causes circulation to struggle to the lower legs.
I agree with the seat angle. We sell chairs at work that are horizontal for aesthetic purposes, and they suck to sit in for more than 20 minutes. The horizontal surface causes pressure points near the coccyx and align the spine in a terrible position. Grandjean recommends a seta tild of 20-26 degrees for an wasy chair and the angle between the backrest and seat pan between 105-110. This leaves the backrest rake at 136, which is really only good for resting, and can be a pain to get out of. Le Carpenteir found a seat tilt of 10 with a rake of 120 to be good for reading and watching TV. One important thing is the backrest contour and angle, as it helps support the spine and taking load off the pelvis. Be sure not to make it too short though, as you want the spine fully supported.
A common misconception is to equate depth and softness of upholstery with comfort. The luxurious sensation of sinking into a deep over stuffed sofa is indicative of an absence of the support necessary for long term comfort in the sitting position.