Between the Ion and Houdini, you can't go wrong, they are both good wind jackets. I paid a lot of attention to selection of a wind shirt/wind jacket because it's probably the most used and versatile piece of gear I have. I've had both and sold the Ion and got a Houdini. 1) the Houdini fabric is more durable 2) the hood on the Houdini has a superior design offering better protection, zipping up to the mouth 3) the fabric is highly breathable and while perhaps a drop less (really a drop) breathable than the Ion, the Houdini fabric is far more water resistant 4) the Houdini's more generous cut works just as well for next to base layer or over a midlayer - you're going to want that versatility in highly variable weather. The cut is forgiving but not huge 5) Houdini waist drawcord further extends its ability to keep you warm 6) while the old Patagonia Dragonfly was very see through, the Houdini is not, if that bothers you.
I also bought and used a Montane Litespeed for more than 6 months. Montane makes a lot of quality products and I had read some reviews pointing to good breathability. No comparison. Houdini wins hands down. This was the deal breaker for me - during a fast hike in the fall, my wife wearing a Houdini and me wearing the Litespeed, with the DWR in the same shape as new, we were caught in a moderate steady rain. I was soaked in 5 minutes - shoulders, back, chest, arms, while my wife's underlayers were dry after 45-50 minutes of rain. In that same time, I was soaked to the bone. The lightspeed double fabric hood is a little warmer - but the Litespeed also weighs about 2 ounces more for that, and in that case, I'd prefer to take my light 1-2 ounce thin powerstretch balaclava. And by the way, my head eventually got soaked through the hood in that rain as well.
I couldn't stand Patagonia's old Dragonfly wind jacket as it was about as breathable and comfortable to wear as plastic. However, the Houdini is an incredibly well designed, functional and comfortable wind shirt.