This totally neglects that in many cases, your smartphone is the closest transmitter. Having many base stations reduces the output power of your smartphone necessary to create the field power at the base station.
So I guess then it would come down to how often your phone transmits. If there is some sort of potential damage from non-ionizing radiation at power levels below those that cause heating damage (which has not been demonstrated), it is probably going to be a probabilistic thing. Probability of, say, cancer will be something like the integral over time of field strength above some threshold.
With the sparse high power tower network, that integral is low when you phone is not transmitting, but spikes way up when it is. With the dense low power tower network, you are trading a higher integral when your phone is not transmitting for a smaller spike when it is.
Figuring out which is better would require a lot more information than anyone here likely has (if it even matters at all...it is not proven that there is any danger in either scenario).
Warnings, but no errors.