I've been to both. Melting Pot is better.
Melting Pot is a great atmosphere and a wonderful staff. It may be a bit more expensive, but they offer endless "dippers". So if you run out of items to "dip" they bring you more, even for the chocolate - and they will bring what you want to dip so you can pick and choose. It is a much classier place too.
Simply Fondue - Not a great location, there was a strung out lady between the parking lot behind the building and the front door - we had our 4 year old daughter with us so that was not so good. I would probably not go there after dark!! When we had our chocolate fondue, the dippers were very limited and if we wanted more they charged for them. You didn't get as big of a variety of dippers either. It was a bit less expensive, but not much at all.
If you want romance, go to the Melting Pot!!
Happy Anniversary!