Fabric: As with most jackets, you want a Harrington jacket to be able to handle a bit of rain and wind. Most options come in cotton or a cotton-polyester blend. Either one can handle a bit of foul weather but it is always best to check out the item description or ask a salesperson before you buy. There are some exceptions, like leather or wool, but most Harrington jackets are lightweight and should have some give to the fabric. If you plan on actually golfing in your golf jacket, then there are options that use synthetic fabrics with stretch.
Fit: Most Harrington jackets are a little form fitting, which is part of the sportswear heritage. The original design, still offered by Baracuta and Ben Sherman, is slim fitting with a rounded off raglan shoulder to allow for smooth range of motion and cut back on fabric drag. Because of this, it is easy to mistakenly buy a size too small if you aren’t trying the jacket on first. So check any sizing charts that are available before making your purchase.
Open or closed waist and cuffs: The biggest variation between Harrington designs is whether or not the cuffs and waist have elastic bands. If you want to stay true to the original design or prefer a tight, bomber-like fit, then go with the elastic. If you prefer a looser, more modern fit, then go with the open cuffs and waist. One thing to note here is that the open cuffs and waist pretty much always come with a collar.