This is a highly debated topic. Some like very heavy ammo, such as .4s. Look up the airsoft trajectory project, and it will become quite clear as to why heavy is better with the sniper rifles. But then there is the very important issue of bb quality. Unfortunately, high-density styrene is apparently difficult to cast with consistent density. This leads to bbs with the center of mass not at the center, which can lead to more inconsistent/erratic flight patterns. Many argue that the heavy weight has a far greater effect, and helps keep the bb steady in spite of this. Heavy ammo is also a bit less affected by wind. Many others prefer the lighter (but still heavier than normal) Maruzen .29s and Bioval .3s, and other similarly weighted bbs with very consistent diameters and consistently centralized weight distribution for their very consistent flight patterns and quality assurance. I recommend trying a variety at some point and seeing what works best for you in your hop-up and barrel setup and field conditions.
dmh078 has it right on the money. It all comes down to personnel preference. I personally like Madbull .36 and .4 bbs. Others like the lighter, but still relatively heavy .29/.3 bbs. Basically, like dhm078 said, try a bunch of different kinds. It will give you the opportunity to see how your rifle performs with each weight bb, and what weight works best for certain situations. For instance, on an extremely windy day, a heavier bb might be necessary/required.
P.S. Watch your "I"s Bottoms, we capitalize them here.
I use madbull .4's and they work fantastic for me. My M24 is a tack driver. Try to stay away from matrix bb's as well, I used their .4's and they had the problem as mentioned above and were overall just terrible bb's.