If you are purchasing the reamers for the first time, if the price is close, get the longer reamer if you can. Straight flute reamers are pretty cheap and will do most of the applications except for interrupted cuts (keyways usually). Spiral is than needed. Most of my holes are 1/4"-3/4" deep. But now I am making bearings for steam engines and the flutes are long enough for that without further purchase. If you are going to do high volume in a particular size, get the shortest/cheapest one you can. Reamers can be resharpened but it is not worth the trouble and it alters the size.
Charles