The only difference between a Matron and Maid of honor is that the Matron of Honor is a married lady, and a Maid of Honor is unmarried.
If you have 2 women in your life that you feel you want to be this close to you on your wedding, then I don't see a problem. Maids of honor normally do some shower organizing, and are the ones that hold your bouquet during the ceremony and make sure your dress is straightened. They also have been kind of the 'herder of cats' typically.
If you have a tie between 2 women for the maid of honor position, you may want to consider the one who can be the most calming, practical and helpful for you.