Hi Greg. I'd go with the general design of having a flip-top bench or something similar. That way you can have both your seating and storage space at the same time.
Would you mind sharing photos? My suggestion would depend on how wide and tall the areas are. I agree with Dan that the flip top is a great solution .. and can easily have cushions on top to dress up the area or give a quaint zone for reading books.
Drawers are incredible too - if space permits. A bit more work behind the construction having to build the drawers and buy all the hardware, but it makes the area really easy to access!
A lot of the window seats I have seen are fairly deep...18 to 24 inches or so. You will want to provide at least that much depth. depending on how high the bench portion is compared to the angle and depth of the dormer opening this may or may not fit. If you have some lumber scraps kicking around you might want to mock up a prototype to see if the scale and position will work first. Nothing worse then planning and executing a nice design to have it be uncomfortable to use.
If you have basebd heat strips there, you will want an open seat bench only. Otherwise a built in with hinged top is good for some storage, but Most folks I have built those for never really use the storage space. It becomes a fill it and forget it place. In one house where I was working, I openned one up to find an electrical subpanel from way back when. The lady had lived there all her life and never had openned it! There were also old fire extinguishers in there. She said "I always wondered what happened to those...."
I think it is because seats like this get cushions and pillows placed on them, an then it becomes a monumental task to move them all aside t get to the interior stored items.
One client had me build chests to place in under dormer windows. Functioned as toy chests, but also to make it easier to get to windows as egress. This was an old house and the windows were a bit high. But being able to move the chest out of the way makes for easier cleaning and washing the windows too. I used some soft close lid hardware so little fingers would not get pinched by slamming lids
Thanks to everyone
Let us know how things turn out there Greg. Good luck with it!