The events are very different and so it really comes down to your personal preference.
Scarefest is more family friendly, with a more fun Halloween vibe. Although it's been considered second to Fright Nights for awhile, last year it excelled due to lack of actors and last minute changes at Fright Nights, with Scarefest not having these issues and having a major overhaul. There are less mazes here meaning slightly less variety.
Fright Nights on the other hand is very much an adult event. The mazes here are all quite scary with a mixture of creepy, intense and strange. Although none are too bad, it really depends on what scares you. If you prefer creepier 'more realistic' experiences then BLAIR WITCH or PLATFORM 15 may be more to your taste. The resort are promising a better event this year, as last year it was pretty disappointing! (CONTAINMENT you must buy a separate ticket for). It should also be noted that 2/5 mazes are getting a major overhaul this year and PLATFORM 15 is an all new experience.
Hope this helps.