If you never wear a dress then I believe it is fine for you to wear whatever you are most comfortable in and whatever makes it you! My husbands mom never wor dresses either, she wore a suit outfit for our wedding and it was very appropriate and very her!
Either would be fine. You will be surrounded by friends and family, so be yourself and be comfortable. I would go with the pant suit and a very dressy blouse. If you go with a dress, any formal wear would be fine, tea or floor length. For either, wear a color that compliments the wedding party colors.
It's accepted convention that the mother-of-the-bride sets the tone for what the moms will wear. She gets first choice of color, then the groom's mom follows with something similar or complimentary. A beautiful pants suit would great, just take cues from the "other mother" and run it by the bride.
Wear what you're most comfortable in. A pants suit is perfectly acceptable for a mother of the groom. It will not be considered tacky or inappropriate. However, if it is an evening wedding you might want to go a little fancier than you would for a morning or afternoon ceremony.