Sword saint is one of the retardations in Kangmaker/Pathfinder. What does a Kensai have to do with magic? It's supposed to be a class focused on martial mastery, skill, technique, etc. Instead, here, you have some fairy casting spells.
And it postpones the most useful arcana by 3 levels.
. The class is basically a gigantic trap which forces you to endure hardships stuck to one weapon hoping for the big reward at the end when your teammates are using all the good weapons you find along the way.
What is the most useful arcana? In my book that would be Ghost Blade (with Brilliant Energy) and Bane Blade. But it's not like you can get those sooner on other maguses. Oh and Spell Recall, but an Eldiritch Scion is missing that as well.
You probably mean Dimension Strike. Well that's nice, but also expensive. I prefer to use those 2 AP on Arcane Strike PLUS Perfect Strike increased Crit multiplier if I want to drop someone fast. Getting +7 Accuracy along +5 weapons should be enough to hit most enemies. You can even increase the odds even further with Brilliant Energy (does a lot of what Dimension Strike does, but lasts minutes per level rather then 1 round) and Shatter Defenses. Only the SS can increase the crit multiplier btw.
If you know what weapon type you want to use, its not a problem. Quite the opposite, you get the proficiency in a chosen, possibly EXOTIC weapon, as well as Weapon Focus in it for FREE at level 1. You potentially save 2 feats, so its actually an advantage. Kingmaker heavily rewards weapon specialization. Weapon Focus may not be that much, but its a prerequisite for Dazzling Display => Shatter Defenses, which can mean you always attack flat-footed enemies. Improved Critical is a Prerequisite for Seize the Moment, which creates lots of Attacks of Opportunity.
As a magus you should probably focus on weapons with somewhat wide crit range, and if you want to do spell combat, you need a one-handed weapon, so its not like the list is very long. Basically choose between a scimitar, dueling blade, bastard sword, rapier, estoc or falcata. I'd recommend one of the first 3, rapiers would be slightly inferior damage-wise, estocs and falcatas have sucky availability. Many scimitars (and only scimitars) have the Mage Blade property, which is very nice for maguses (a temporary +5 15-20/x2 weapon at level 6? Yes, please). Oh, the Sai is also a solid option for a sword saint with a monk dip. Up to 4 bonus attacks at max AB -2 with Flurry, Spell Combat and the unique sai with Greater Speed? Yes, that does sound pretty neat.
Actually as a Sword Saint you get lots of free great feats: Exotic Proficiency, Weapon Focus, boosting Initiative with Int (better then Improved Initiative), Increasing number of AoOs with Int (simiar to Combat Reflexes), confirming Crits with Int (better then Critical Focus). Also you get to add Int to AC and, eventually, also to damage vs flat-footed enemies. You even qualify for Greater Weapon Specialization.
Sword Saint is also the best magus tank, by far. Less spellcasting capacity, far more martial prowess. A more then fair trade IMO. Definately NOT a trap.
