The WiFi portion of routers are funny creatures and are not all created equal.
I would tend to favor a separate user purchased router over an internal Modem/Wifi Router "combo" unit for a number of reasons.
Most routers perform best in a central elevated position. This can be very hard to do with an entire Modem.
Wifi Modems do not have external antennas that can be tweaked to improve directionality.
A number of consumer routers allow data tracking per day, per device, per application
Here are some tips for getting more range out of your router (ours, or a third party router). Depending on your setup, even an inexpensive repeater might help. I also recommend thinking about whether something could be causing interference. In my home, for instance, my wifi takes a big hit if the clothes dryer is running!