What stat is better for Magus, strength or dex? I'm having trouble deciding which to choose for my build. I want to use a longsword even though I know it is not optimal because of the 19-20 crit range (is it really that big of a difference compared to a 18-20 weapon such as the scimitar?) but I'm worried about feat starvation with the dex build where as the str build looks more flexible.
You can’t do a good DEX build with a longsword because it doesn’t work with weapon finesse. The reason people prefer scimitars and rapiers and estocs for Magus is that they’re finessable so you can use DEX instead of STR on your chance to hit.
A longsword is just as finessable as a scimitar, you just need Slashing Grace.
People are using estocs, scimitar, or rapier because your spells have the same crit range as your weapon, so you get more Shocking Grasp crits with those swords.
So what happens if you use a weapon with a different crit range than your spells, like a longsword? Does it reduce the chance for your touch spells to crit to 19-20 or does the spell still have a 18-20 crit chance and you just crit less with the actual weapon strike?
Also, what are you opinions on the magus strength build? I would assume the damage would be somewhat stronger since you could use power attack and 2h your weapon for the 1.5x bonus.
Your spell crit range is identical to whatever weapon you're using at the time.
You can get Piranha Strike for Dex builds which operates the same as Power Attack, but doesn't qualify you for all the feats that Power Attack does.
You don't want to 2-hand, going 1handed and spamming your Touch cantrip will get you an extra attack every round with a 1-handed weapon.
I do like Str builds though, they're probably a bit better late game but weaker early on when your AC is worse. You can mitigate it once you get Blur and Mirror Image though. It also gives you more room for feats since you don't need Finesse, Weapon Focus, Slashing/Fencing Grace (if you go Rapier you can delay Fencing Grace, there are a bunch of good Agile rapiers in the early/mid game but you probably want it eventually), and you won't be as locked in to one weapon.