fan info help

i have a fan that is 120 mm with 1500 rpm i am looking to buy a fan but can't decide between a 80mm with 2200rpm or a 120mm with 1800 i want to know which is better also in cool what is more important for cooling size or speed rpm

They will give you almost the same cooling performance, I'd go with the 120mm because it is probably less noisy.
Where is this fan will be placed at?

If your case will accomodate it, a larger fan is always better than a smaller one. Due to the larger surface area of the 120mm fan when compared to that of a 92mm or 80mm unit, the same amount of air can be drawn through it as that of a smaller fan, even though it is running at a slower speed. This has the advantage of reducing system noise. The important figure to note is the CFM (Cubic Feet per Minute), and it indicates the volume of air that can pass through the fan when it is operating as per its rating.