Basically the bigger restrictor (8mm) will give you more of everything except economy. So if you want raw power and have no concerns about run time or smooth power control, the 8 mm is the ticket.

Conversly the 6.5mm restrictor will give you a much smoother motor. You still get good power, but itīs not as brutal as the 8mm and you will get better run times. You will loose a little bit of top end, just by the nature or the smaller inlet size restricting total air flow into the motor. On some tracks it does seem as though the 6.5 mm restrictor actually boosts bottom end repsonse at partial throttle openings, due to a ram air effect caused by the more tapered throat of the smaller restrictor, but this effect is pretty negligible.

Most people go for the 7mm in truggy, which gives a nice compromise between power, smoothness and economy.
Buggy either 7mm or normally 6.5mm. Most buggy drivers like the smoother power of the smaller restrictor, plus the added run times with the buggy gearing. The power offered by the bigger restrictors is normally lost in wheelspin and lack of control in a buggy.