900 MHz vs 433 MHz for urban environments
433MHz will tend to be a bit better IMHO. It's the ability of the two frequencies to sneak between all the obstacles in our modern environment that is probably about equal but, at approximately half the frequency 433MHz will have more penetration due to the free-space link-loss formula. Link loss (attenuation in dB between transmitting antenna and receiving antenna) is: -

32.45dB + 20log\$_{10}\$(MHz) + 20log\$_{10}\$(kilometres)

At 500m, the link loss for 915MHz will be 32.45dB + 59.2dB + 0 = 91.7dB

At 500m, the link loss for 434MHz will be 32.45dB + 52.7dB + 0 = 85.2dB

Not much in it but 5.5 dB is better than a kick in the crotch!