We also fly out of SLC and I have been watching airfares for the past year or so. I think the Frontier deal you found is good, I would probably jump on it if you need those specific dates or if you don't want to wait or watch the airfares for the next few months. We actually booked into Tampa right when the dates opened because it was so much cheaper (at the time we booked on SouthWest we paid $266 roundtrip vs. $466 into MCO). I know Southwest was having a deal about 6 weeks ago where the price was $240 roundtrip from SLC to MCO around those dates you mentioned, but I think that was a really great deal and might not be repeated. I do think the Southwest flights will come down in price before your trip, but it's hard to say how much.
Airfare prices have definitely climbed since your last trip in 2009. For us, anything under $400 is good and under $300 is a steal. Just my opinion though! Good Luck!