I have personally owned both the Hahn and DDMWorks intercoolers. Both are tip top quality. I personally never had any fitment issues with both. The Hahn(4") in regards to size is .5 bigger than the DDMWorks(3.5") and has nicely curved endtanks for better flow. The DDMWorks though offers a really COOL(hehe) feature......Extruded core. They have a better description on their website but instead of having traditional bars going across like most intercoolers(like the hahn) you have rounded tubes/pipes. The efficiency of removing heat is therefore increased as you have more surface area exposed to the incoming cooler air. This also aids in the way the intercooler flows.
Anyways, it comes down to your setup.... if you plan on getting a larger Turbocharger then going with the Hahn is the way to go. Getting the Hahn and sticking to the stock turbo is fine but a little overkill. Some people will think " Well its bigger so better cooling, right" Well to some degree yes but not necessarly. You also need to think about that K04 filling that bigger core, not that it can't as I had the Hahn on a K04 without a problem, but just something to think about as it can effect booting times vs stock or the DDMWorks. This is why I say that a larger turbo capable of flowing more CFM would match perfectly with a Hahn.
The DDMWorks is slightly smaller and boost times(though small) will be slightly better as it doesn't need to flill as much as a larger core. Safe to say though that larger Turbo's have been used on DDMWorks intercoolers with awesome results.
Let me also throw this out there. The stock intercooler is actually a well designed FMIC for being what you get from the factory. The only time an issue arises is when you run the car hard(ie Road Racing at a Track or multiple passes at a Drag strip) thus having an aftermarket intercooler becomes a good investment. Other than that, you'll be just fine if your going on road trips or just meets(Car shows) with the stock OEM intercooler.
I'm putting in a Hahn IC today but I believe any of the aftermarket IC's are going to be quality made. I've not seen much in complaining of any aftermarket IC.
Hahns price is really good right now. I'm slightly upgraded and consider plans for a bigger turbo (not decided how big) and adjusting some internals to handle that power. The Hahn IC will overkill anything I do so if I keep going bigger, I'm set. Oh and the price was really good. :yesnod: