How many air changes do you want? Assuming you have an 8' ceiling, 250 CFM will give you a bit over 2 air changes a minute, meaning the fan will remove a volume of air double the volume of the space every minute. Bear in mind that the more tempered air you exhaust, the more make-up air you will have to heat in the winter, and cool in the summer.
All other things being equal [which they rarely are], greater air volume means greater noise level. Spending a bit more on a fan with a lower sone rating may be worthwhile in the long run, no point having an exhaust fan if it's so loud you can't stand to use it. Almost every kitchen exhaust fan I have encountered over the past 40 years fell into the too loud to use category.