I would say that the ranges at which you would use a open sited gun in 30-30 ( less then 150 yds, most likly less then 75) you would not really see a diffrence in the two if you used simlar bullets.
I used a 30-30 for years and yes, they have been one of the most prolific deer cartridges mainly because the Winchester 1894 was chambered for the 30-30 Winchester and they still sell an equivalent rifle today. I would never take a shot outside of 100 yards with mine but in the northern Michigan woods, you didn't need too!
The differences are major between the two calibers and here's the numbers right off the Federal ammo site.
30-30 Win, 170 gr - Average Range 100 yards - Velocity is 1900 fps @100 yds - Energy of 1400 ft/lbs @ 100 yds
.357 Mag, 180 gr - Average Range 25 yds - Velocity is 1000 fps @ 25 yds - Energy of 400 ft/lbs @ 25 yds - Bullet Drop @ 100 yds is 9" and 10mph cross wind will push that bullet nearly 4"...
There is no comparison. Obviously barrel length will make some differences but outside of 25-30 yards the .357 loses too much energy and it drops fast!