Fan speed does not matter. It is related both to noise level and to air flow rate. If you have noise level in dBA, lower is better (less noise), and differences of one or two don't really matter. If you have air flow as CFM or m³/hr, especially if it tells you that at different speeds, use that data - more air flow is better. (Conversion: 100 m³/hr = 58.86 CFM (ft³/min). You don't need speed if you have those specs.
You may also see specs for fan pressure. The pressures involved are very small. What they really are for is the type of resistance to air flow (or backpressure) the fan is designed to blow through. Some things like heatsinks on the CPU consist of many closely-spaced fins the air must pass between, and these small air channels resist air flow more than open space inside a case. So fans optimized for use on a CPU heatsink have higher pressure ratings for a given air flow rate than fans for other applications. By the way, any INTAKE fan should have a dust filter in front of it, and these do offer some air flow resistance, but not enough to worry about. That is, as long as they are clean! You MUST check and clean air dust filters periodically, or you risk reducing case cooling due to a plugged dist filter.
IF you use the "Automatic" fan speed control system of a mobo SYS_FAN port (or CPU_FAN port for the CPU cooling system), note that it really is NOT a "fan speed control". It is a TEMPERATURE control! In each case the control system has a set temperature target and a sensor that measures the actual TEMPERATURE at a particular point. It manipulates the fan speed so that there is sufficient cooling to keep the Temperature at the proper target. It really does not care what the speed is - it will manipulate the speed to anything it takes to get that Temperature on target.
Now, fans DO generate speed signals to send back to the mobo for measurement and display, but the automatic temperature control system does NOT use that information for its function. However, other mobo systems check that speed signal, display it for your interest if you wish, and make sure that it never drops off to zero. If it does (or, if there's a bad connection and the signal fails to reach the mobo) it generates a warning message just so you know your cooling system is failing and needs repair. In the case of CPU fans, some mobos even take more drastic action if the CPU_FAN speed fails. But that is not part of Temperature control, it is part of component failure monitoring.
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