Either is fine.

The CO2 leak-proof tubing is fine, but is expensive.
PVC tubing (the clear kind that is normally used for air pumps) is cheap, but becomes brittle with time.
Silicone tubing is a little more expensive, but remains more flexible with time. It is slightly more permeable to CO2 than PVC tubing, but still negligible.

I use silicone tubing because I had it handy, and I like how it stays flexible.

 

If you don't mind a slightly thicker tubing, you can go to a hardware store or homedepot and get a coil of 1/4 OD 1/8 ID clear vinyl tubing. It's much cheaper and the thick wall means it will be much more co2 impermeable. It's stiffer than aquarium vinyl so sometimes it helps to warm it up with hot water to slip it on to a hose barb.

 

search on e--bay "Tygon tubing ". Aquaticmagic on e--bay also sells Clippard Co2 tubing.

 

I hate working with co2 tubing as they're so tough I have a really hard time removing it from whatever its attached to when I need to. I found just regular air tubing to be fine and a lot easier to use. I haven't tried silicon tubing yet but will give it a try if the regular air tubing becomes bad over time.

 

1/4" OD by 1/8" ID is standard. LFS airline tubing has a thinner wall.

This is a popular type (I use it as well):
http://www.usplastic.com/catalog/item.aspx?itemid=38587&catid=864

 

I use silicone airline tubing. I have used airline tubing for a long time with no issues. The only tubing I got that was meant for CO2 was the grey ADA tubing which got brittle really easily. I had to cut it shorter after removing it every time or it would pop off.

This is exactly what I use. Tends to last the longest of anything that is readily available:
http://www.petmountain.com/product/aquarium-pond-airline-tubing/11442-501706/hagen-elite-silicone-airline-tubing.html?utm_source=googleproductads&utm_medium=cpc&utm_term={keyword}&gclid=COungZLI-bMCFSmCQgodTkwAbA

 

I was at the home depot yesterday and they have clear plastic tubing 1/4" OD and .170" ID. Not sure if this is the same, but I guess bringing along a airstone to try the fitting would work.

 

Any silicone tubing will be fine if you are using a regular glass diffuser with 30psi but if you are using one of those GLA atomic diffusers you will need the real co2 tubing otherwise it will leak.

 

The stuff at Home Depot in the plumbing aisle is just fine.

I've been using it for a long time because it's about 1,000% cheaper than the cheapest stuff at the LFS.