I don't know magus builds intimiately, but...
If you're a dex based magus build then certainly it's better to use arcane mark to effectively get another attack, because your relative lack of strength means your weapon damage is the majority of damage you can cause without spells.
If you are a strength based magus, with power attack (which I don't think is a normal feat for magi), then two handing a weapon gets you an extra point of damage and you'll get 1.5 strength. Depending on the exact weapon you're using and the average damage it deals it will probably be better to swing it two handed.
I would take arcane strike instead of power attack and if I do this it very much IS worth it to make the spellcombat with whatever you have.
But I prefer being a hexcrafter and using brand over arcane mark. That way, if your target fails his save he's dealt another 1 point of damage.
Depends on the opponent and situation, I guess. At the cost of a single cantrip, I don't see why you wouldn't at least have it as a possibility.
So if I think I'm going to hit either way, I'll likely go for the spell combat and try for more crits (assuming i have a scimitar or katana). If up against an opponent where every point on the attack roll counts, I'm likely to go 2-handed (and possibly avoid using power attack, to boot).
Ok , basically the questions comes down to is it better to use spellcombat with arcane mark (to fight like a two-weapon fighter) or is it better to use use your weapon two-handed???
It is better to use spellcombat with arcane mark.
The DPR is higher for making two (non power) attacks with spellcombat than a single attack with extra damage from 1½ str and PA.
The more additional bonusses (such as arcane strike) you can find, it become even better.
Interesting question.
Assume a +1 longsword (could be bastard, or some other one-handed weapon), and L.3 (can't take Power Attack at 1), and Focused Mind trait (and not Combat Casting feat):
BAB+2, Str+4, +1 magic, +1 misc (bard, bless, flanking, etc.), [-1 P-Atk] = +8 [+7 w/PA]
Damage: 1d8+5[7], Two-handed: 1d8+7[10]
Concentration: 3+3+2 = +8
Arcane Mark:
Concentration DC 15, succeed 70% of the time, so 1.7 attacks on average
1.7 attacks at +6[5] for 1d8+5[7]
Two-handed:
1 attack at +8[7] for 1d8+7[10]
Expected damage vs. various ACs (add 10% to include crits, but they make no difference between models really):
AC - AM - AMP - 2H - 2HP
10 - 13.7 - 15.6 - 10.9 - 13.1
14 - 10.5 - 11.7 - 8.6 - 10.2
18 - 7.3 - 7.8 - 6.3 - 7.3
22 - 4.0 - 3.9 - 4.0 - 4.4
26 - 0.8 - 1.0 - 1.7 - 1.5
Enjoy! Remember, this is the average damage - some of those values are a lot more random than they appear (the sort of difference between doing 10 damage 50% of the time and 100 damage 5% of the time).
There, honestly are very few times where you don't want to be using spell combat. You get 2 attacks both using a reasonably good weapon (Assuming just a longsword for the strength build, or scimitar for the Dex build), and you get two applications of your strength damage, instead of 1.5. While I am more familiar with the Dervish Dance build, I don't know how common PA is on Magi, but even so, you're getting 2*Power attack bonus, rather than 1.5 times that bonus, for two handing. Only if you're very inacurate (i.e. hitting on a 16 or greater) is two-handing better (assuming a longsword and the stats you have there; if you have lower strength, you need to be even more inaccurate to be worth it.)
I've been playing with a magus lately
It's that class I always drift back to when designing a character but have yet to play...
So far i'm a half-orc 1 sorcerer (Primal/Draconic) 1 Oracle (Flames) 15 magus 3 fighter
Fixing my BAB issues at the end, getting +2 damage per damage dice on a fire spell, and 1/2 my level for that spell. (Metamagicing snowball to be fire, and taking a trait to negate it)
Something like 5d6+17 as an additional damage, It staggers and enflames my enemy.
as part of a weapon attack.
It's the only magus cantrip that works in conjunction with Spell Combat... giving them the freedom to Spell Combat at will all day long.
EDIT: functionally, it's a free (albeit slightly modified) TWF
Can I ask what benefit Arcane Mark gives in spellcombat? It doesn't seem to do anything but let you track someone, since it has no status and does no damage, so all you are doing is poking them. Am I missing something?
Spell Combat combined with spellstrike gives you an extra attack, if you cast a touch spell.
Arcane Mark is a touch spell, which you can cast as often as you like, since it is a cantrip.
My gut feeling non-mathy short answer - Would you need to cast defensively? If you do then it's not worth it. If you have reach or can use 5' steps appropriately so you don't need to cast defensively then yes it's worth it.