Traditional dress etiquette dictates that the mother of the groom should wait until the mother of the bride picks out her dress. You want to make sure that you end up in similarly formal outfits. The mother of the groom should not wear a strappy beach dress if the mother of the bride is in a ball gown.
While not all brides follow etiquette rules, you may not know which ones the couple feels strongly about until you ask. Depending on the style, you may also be able to wear a fancy pantsuit to the wedding, instead of the traditional mother of the groom dresses.