Hmm, that's a tricky one. You would "think" that you would be able to do much more on seated, simply because
A) You don't have to really support the weight with your lower back.
B) 150 pounds with pulleys is less resistance than 150 pounds without them.
However, it really depends on your particular machine that you use. The one at school has 3 pulleys, while the one at the gym here at home only has 2. Obviously the more pulleys, the more weight you'll need to use.
Sorry for beating around the bush here... if it helps, I used ~150 lbs. on the seated rows while I did them, and currently am at ~165 lbs. on the pendlays. I'd say for your ratio, they'd be pretty close.