http://te4.org/wiki/Combat_Damage
Read this, it has a lot of good info. But I'll break down the important bits here for you.
Base weapon damage is the square root of the lower damage number on your weapon divided by ten, then add one to that number. Divide the whole thing by two to get your damage.
(sqrt(base_dmg/10)+1)/2
Physical power is a tiered stat, meaning you get decreasing gains as it goes up. Tier one is equal, so 5 physical power gives you 5 physical power. However, in tier two, it costs two points, so 5 physical power gives you 2.5
actual physical power. So on and so forth.
Tier Final Values Cost per point
Tier 1 0 to 20 1
Tier 2 21 to 40 2
Tier 3 41 to 60 3
Tier 4 61 to 80 4
Tier 5 81+ 5
Finally, your total damage is determined by a couple of factors, but the important bit is the relative modifier between physical power and and weapon damage. The formula is dam=0.3*(phys_power+totstat)*power*talented_mod.
So, to crack out my calculator, it looks like...
Greatsword (52.0-83.2) has a modifier of 1.640175425
Greatsword (37.0-59.2, +16 Physical Power) has a modifier of 1.461769203
If these are Stralite, which would make sense given the damage, I can assume your strength is around 50. This gives your physical power a modifier of +60 before being multiplied.
So in order to determine which is better, we plug it into the formula, ignroing the numbers that don't matter.
1.640175425*(60+physical power)=1.461769203*(60+(physical power+16))
This would be easy, except physical power is tiered. So let's simplify it a bit.
1.640175425*60+1.640175425*physical power=1.461769203*60+1.461769203*(physical power+16)
1.122048147=1.461769203*(physical power+16)-1.640175425*physical power
1=1.302768698*(physical power+16)-1.461769202*physical power
Much better. Now we just have to plug in physical power formula, which is a bit trickier. We have to treat it as a piecewise function.
0<x<20, physical power=x
21<x<60, physical power=20+(x-20)/2
61<x<120, physical power=40+(x-60)/3
121<x<200, physical power=60+(x-120)/4
Now to figure this out finally, all we need to do is find what x makes the formula above true. This is where the two weapons are totally equal. Higher than that, the sword without physical power is better, lower and the sword with it is superior.
The break point is just shy of 29=x. What does this mean in terms of helpful advice? Plug the x in, and you get 24.5 for physical power. This means if you have 24 or less physical power, take the sword with physical power. If you have 25 or more, take the sword with the higher base damage.
Important note: This all assumes you have a strength of 50. This does affect the result, but unless you have a wildly higher strength, it shouldn't matter too much.
The actual answer, skipping all math bullshit.
If your physical power is 24 or lower, use the sword that has +16 physical power. If your physical power is 25 or higher, use the sword with extra base damage.
And next time, please list your strength stat. That would let me tell you with 100% certainty what would be better for you in terms of pure damage.