As stated, it is up to you what you want to wear for formal night. Either of your outfits (suit or dress) should be fine. My table mates weren't super dressy, but dressy enough, if that makes any sense. There will be 2 formal nights for your cruise. Check your cruise compass, which will be delivered the evening before your next port of call, or day at sea. On my cruise, the formal nights were Monday and Thursday. This was the Western Caribbean itinerary. I don't know if it's different for the Eastern Caribbean.
I'll post a review later.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)