I seriously doubt that any wireless router has any more power than any other. The maximum power output is regulated by Federal Law. Different channels, different speeds (based on the different standards; 802.11b/g/n). But, just as Citizens Band radios had signal boosters available, you can buy those for a wireless router (probably along with similar problems caused by the CB boosters).
Most people simply use multiple routers to spread the signal around a house.
I doubt that Charter (or Comcast, Brighthouse, etc.) ever recommends any other brand of router, they simply aren't in that business and most of their techs simply don't know Apple exists. My opinions of Radio Shack have been published here before. Suffice it to say I don't consider their 'sales' people to be experts at anything, much less electronics/computer/WiFi advisors.
What you bought works. That's all that matters. For more ways to strengthen your signals, read the tips at <
http://www.wikihow.com/Boost-a-Router-Signal>.
As for the 'dangers' of sitting next to a wireless router, your cell phone has about twice the output. And there are still no definitive studies supporting any danger of talking on one of those 24/7 (other than the banal 'conversations' that would entail!).
More info on signal strength can be found at this <
Wiki page>.
