In my opinion, Sabertooth is better than Fairy Tail. Sabertooth has the twin dragon slayers, which are third genoration and are actually good and interesting people. They help their friends in need and are friendly. Despite being rivals they are also friends with people at fairy tail.
I will be comparing the Strength of members. While sabertooth has the twin dragons among other stand out members Fairy Tail as a guild has members that have breached the ranks of and even beaten members of the ten wizard saints which are some of the strongest wizards in all of fiore. If one is to compare the dragon slayers in the guilds the twin dragons are outmatched by natsu and gajeel. If you recall they even lost to natsu on his own in the grand wizard games.
Even if they are fairy fail has some good strength, sabertooth has some of the strongest mages like Minerva and Rogue and Sting when using their combined powers. To be fair Fairy Tail is a bunch of Perverted strippers that always destroy their own guild hall. Sabertooth may not have the smartest people *cough cough * sting *cough cough* but most are pretty smart. Fairy Tail only won the grand magic games because Sabertooth had not yet considered anyone in their guild as a friend (as they do now)
Fairy Tail's mages are by far and wide the strongest on fiore due to the feats they have done such as defeating all the members of tartaros. Their intelligence in some situations is questionable however they also have brilliant strategists such as makarov and mirajane. Lets talk about the creators of the guild, Mavis vermilion the creator of fairy tail created one of the most powerful spells in the entire magical world when she perfected what is known as fairy law.