Mercenaries is a bit of an odd-ball for a faction - it handles like 4 mini-factions in the same faction, and that makes for some very tight restrictions on what units you can have in a particular list.
The four mini-factions are Searforge (Rhulic dwarves), Talion (Pirates Arr!), High-Born Covenant (resistance fighters from Llael), and Four Star (The most generic mercenaries faction). Some models are only usable for a particular mini-faction, some work for a few. Usually you'll find Rhulics and Pirates are very restricted, but most of the models will generally also work for either High-Born or Four Star, as they're more diverse groups. For instance, Durgen Madhammer is a Rhulic Dwarf warcaster, but he'll work for Highborn or 4 star as well as Searforge. Then again, his own rules only allow him to take Rhulic jacks, even if you take him in a 4* list, which further restricts his choices.
Each caster also has their own Theme list they can take, which sometimes allows for very different sets of units to be taken. For instance, Magnus the warlord (a cygnarian Traitor) has a theme list called the bad-seeds, which allows him to take a certain number of Cygnarian units (basically defectors).
So basically, list selection is a little harder, but mercenaries as a faction is certainly fun and viable.
Meanwhile, Circle are basically a super fast glass cannon. Fast, lightly armoured, lots of movement shenannigans, lots of tricks. They have two general sorts of beasts - their living beasts (warp-wolves, satyrs ect.), who are generally super fast and very lightly armoured, and their large stone and wood constructs, which are generally quite a bit slower, but can take a hit and dish it back.
Both are great choices, though I would probably suggest circle to start with. If you can, try to get a game in at a local store, and just learn how the game works. Also, unless playing against absolute beginners yourself, expect to lose a lot when starting up. It's the nature of the game that it's more tactical, that it requires more finesse in using each model effectively, so until you learn the ins and outs of your force, and others, you'll be at a distinct disadvantage.
Hope that helps