Why can't you vent through the roof? It's relatively simple to do.

Just because it's an interior wall doesn't mean you can't vent outside. The vent doesn't run through the wall, it runs up and then runs between the floor joists. My rangetop and vent are on an interior wall. It runs straight up and then makes a 90 degree turn and runs between the floor joists and out the back of the house.

Recirculating hoods are useless. I don't have an personal experience with downdraft vents but haven't heard much positive about them.

When I began my renovations I was told by my KD and GC that I and exterior vent was impossible and that I would have to use a recirculating hood b/c my cooktop was on an interior wall and the MBA shower drain crossed the joist space I needed to use. I refused to accept that and insisted we come up w/a plan for moving the plumbing. After much discussion back and forth (and me telling them recirculating was not an option over & over), I finally went outside to have the drain moved. They proceeded to come up w/other excuses, all of which I countered and eventually I got my exterior vent.

My duct runs up the wall, makes a gradual turn to total 90o and runs across the kitchen (~11') to the outside wall.

Bottom line...you can vent to the outside from an interior wall....up through the roof or across a room.

Given the choice between a recirculating (ductless) hood and a ducted (vented to the outside) downdraft, the downdraft would definitely win. It really comes down to choosing between the lesser of two evils. Anything vented to the out of doors -- is without doubt, the lesser evil.

As suggested in the posts above, there's probably a way to run ducting either up through the roof, or out through a wall. A good ventilation contractor will almost certainly be able to suggest a way. It may cost you a bit more near term, but in my estimation, a true (outside-vented) range hood would be worth whatever it costs. Recirculating hoods do nothing to remove humidity and heat from the kitchen, while downdrafts are always noisy and not nearly as effective as a hood.

I agree, downdraft wins. I had one in the kitchen I tore out and I did not care for it. But the apartment I'm in has a recirculating hood and it's a total joke.