Carbon steel. Many will tell you to get a wok with the D handles (which has its merits), but I would advocate for a single, long handle. During the cook, the handle can sit outside the Egg and stay relatively cool, such that you can grab it and give the wok's contents a flip. Also, when dinner is ready, you can bring the wok into the house one handed. 

Like @DoubleEgger suggested head over to Tom at the CGS. Years ago I bought my 16" round bottom hammered black steel wok PLUS a "spider" from him to rest on. The spider is unique in woking since with legs down you food cooks hot and fast, but revered with legs down you have more control cooking additional ingredients. One last hint...with a Large BGE bring your egg up to temp with the wok in and then when starting to cook, close the bottom vent, open the dome and you can easily cook your wok meal without fear of an inferno fire! Trust me - I know what I am saying and love my BGE for woking!

theyolksonyou said:

16" hammered carbon steel. I prefer D handle and a good pair of gloves. 

I did this from the wok shop in San Fran based off previous threads. We do stir fry almost every Sunday and it's amazing. I even left it outside for a couple of weeks in Houston. Basically "ruined" it with rust. Several hours of elbow grease later I re seasoned back to new. 

http://wokshop.stores.yahoo.net/wokwi2stha.html

i did the flat bottom. 

I have the 19" wok with the spider(for the xl bge) and have stopped using it since I bought a Blackstone.  It can do the same thing and be ready in 5 minutes. At this point my wok just takes up a massive amount of space on the shelf in the garage. As I get older maybe I can repurpose it to stop the government from reading my thoughts? Always looking out to ultipurpose cooking gadgets! 

nolaegghead said:

Carbon steel - stay away from CI, doesn't transfer heat fast enough.

Carbon steel woks are far more widely available and probably the best choice for many folks and purposes.

However, there's nothing wrong with an Asian cast iron wok tho. Millions of Asians use them.

Non-Asian cast iron woks are thick and heavy and not ideal but not without their place.

i have a bunch, i find the 14 inch carbon steel long handle cheap wok the easiest to use. any bigger and i have a hard time with it. i think it was 12 bucks in bostons china town. round bottom shovel spatula and the aluminum cover are great addons, the cover fits my 12 inch skillet as well.

@Mickey -I've been wanting to do whole fried fish, is that corn flour you used?

Check out http://www.wokshop.com/. They will have whatever you want. The round bottom is more traditional and works very well when deep frying. Carbon steel will season the best, but will require more maintenance and care over a SS version. 

Some important distinctions courtesy wiki:

Materials

The most common materials used in making woks today are carbon steel and cast iron.[2] Although the latter was the most common type used in the past, cooks tend to be divided on whether carbon steel or cast iron woks are superior.[further explanation needed]

Carbon steel

Currently, carbon steel is the most widely used material,[6] being relatively inexpensive compared with other materials,[7] relatively light in weight, providing quick heat conduction, and having reasonable durability. Their light weight makes them easier to lift and quicker to heat. Carbon steel woks, however, tend to be more difficult to season than those made of cast-iron ('seasoning', or carbonizing the cooking surface of a wok, is required to prevent foods from sticking and to remove metallic tastes and odors).[2] Carbon steel woks vary widely in price, style, and quality, which is based on ply and forming technique. The lowest quality steel woks tend to be stamped by machine from a single 'ply' or piece of stamped steel.[2] Less expensive woks have a higher tendency to deform and misshape. Cooking with lower quality woks is also more difficult and precarious since they often have a "hot spot". Higher quality, mass-produced woks are made of heavy gauge (14-gauge or thicker) steel, and are either machine-hammered or made of spun steel.[2] The best quality woks are almost always hand-made, being pounded into shape by hand ("hand hammered") from two or more sheets of carbon steel which are shaped into final form by a ring-forming or hand-forging process.[2][8]

Cast iron

Two types of cast iron woks can be found in the market. Chinese-made cast iron woks are very thin (3 mm (0.12 in)), weighing only a little more than a carbon steel wok of similar size, while cast iron woks typically produced in the West tend to be much thicker (9 mm (0.35 in)), and very heavy.[9] Because of the thickness of the cast iron, Western-style cast iron woks take much longer to bring up to cooking temperature, and its weight also makes stir-frying and bao techniques difficult.[2]

Cast iron woks form a more stable carbonized layer of seasoning which makes it less prone to food sticking on the pan. While cast iron woks are superior to carbon steel woks in heat retention and uniform heat distribution, they respond slowly to heat adjustments and are slow to cool once taken off the fire.[2] Because of this, food cooked in a cast iron wok must be promptly removed from the wok as soon as it is done to prevent overcooking.[2] Chinese-style cast iron woks, although relatively light, are fragile and are prone to shattering if dropped or mishandled.[2]

I'll throw this in to muddy the decision. I have two round bottom CS woks with D handles I have used for over 30 years - I like them both. Last summer I was in SF and stopped by the Wokshop. I saw one of the CI woks and bought it just because I wanted to have one and try it out. It's all I use now on the Mini Max... So what does that mean? I think it means get what you think you'll like - because it will work and turn out great food. The CI wok did season more quickly and easily but that's not a big deal.  Get it hot and wok on. 

Picked up mine at wokshop.com.  18" CS...storage was a problem so I hung it on a wall near the kitchen.