Simpson Duravent Pellet Vent Pipe

Told by my dealer (the only Quad dealer on LI) that I would have to remove old prefab insert and piping up wood chase before they installed a 'flex liner' which they have done 'many times'. After advice from this forum, and verification from the manufacturer (Quad) I questioned my dealer who took a day to research then called me back to apologize for the misinformation and discuss the new install recommendations. Now they are saying that they will be installing a 3 inch solid vent pipe 'Simpson Duravent' with 3" storm collar/3" vent cap (price $588). Is this correct for a Mt Vernon AE pellet insert (price 4143). Is there any benefit to using 4 inch piping? Why is it that they just couldn't have installled the flex liner into my old prefab piping? Thank you to all that helped me prevent having the wrong installation done (price $1590 to remove old insert/pipe, install new, reframe rough opening)! Also any comments on prices appreciated!

All I can say have them install the Duravent Pro, it's a better pipe than just Duravent.
The issue of 3 or 4 inch has to do more with length and # elbow's.
The MV on line manual has the chart. The 4 inch would not have to be cleaned as often. That is a big stove, if it was mine I would go with 4 inch, but that's me.