If your problem is really signal strength and not signal quality a extender may help. You would have to find a place you get good signal and place the extender there but close enough for you to get signal at the remote end. Of course the key problem is you do not always have power at the optimum location for extender.
Most large ISP use pretty well known and reliable modem/routers. You will find little difference in the signal levels between devices because it is regulated by the government. Most the difference are related to firewalls or other user features.
I would first before you buy anything try the standard change the channel trick. The channels operate at different frequencies and are blocked differently. It would also eliminate interference (which is the problem when you have good strength but poor quality).
I have a wireless modem but its a sucky one. It was one of those free ones from verizon. Anyways, I don't get good signal in certain places in my house, so i am interested in knowing which would be better a wifi range extender or a good wireless modem. If you guys have any suggestions on which one and if you know a brand that would be great.