You have to shoot both, and see which is the most accurate, and if either are sufficiently accurate for hunting at the range you wish to be effective.
For example, I have a TC New Englander which loves TC Maxi-hunters, and patched round ball, but hates TC Maxi-balls. You'd think either conical from the company that produced the rifle barrel would work well.... but in testing my specific rifle...nope.
If you are shooting a .490, patched round ball well from your rifle, with a good load, as rodwha suggested... something like 70 grains or higher, the only thing you will get with shooting the heavier conical is a sore shoulder (imho) At 100 yards or less the 177 grain .490 will get the job done.
The patched, round ball isn't "just barely" getting the job done, and the conical isn't a "guarantee" of success purely because it's 40% heavier. The conical was invented for fast reloading for the military in combat. As late as 1867 the experts in big, dangerous game hunting were still using the patched, round ball.
LD