What's the mileage on each of those bikes? The amount of wear and tear each has endured factors into how much they're worth. If both are running properly, the 900 is going to have quite a bit more torque. For solo rides on twisty country roads, the 750 would be more than adequate, but if you're going to be doing group rides or two up, it might feel short on power. All the '99 models got an improved charging system, so the 750 should be a little more reliable as far as the electricals go.