Comcast customer here. Several months ago I reached the same conclusion as OP. Purchased and installed:
1. Motorola SurfBoard SB6141 DOCSIS 3.0 Cable Modem - Retail Packaging - White ($87 from Amazon at the time). After receiving it, I called Comcast tech support (very competent folks), and they walked me through the authentication process (you have to call them, otherwise Comcast won't recognize the new device even if you plug it into your cable connection). Then I returned the old rented modem back to Comcast, and they removed the monthly modem rental charge from bill. Very happy with my new modem and the resulting savings (~1 year pay back period since Comcast charged $8 rental fee at the time).
2. ASUS RT-AC66U Dual-Band Wireless-AC1750 Gigabit Router ($157 from Amazon at the time). This is a powerful piece of equipment, especially useful for apartment buildings where there is a lot of interference from other WIFI routers that your neighbors use. It took me some time (~2 hours) to configure the router properly, but once that investment was made, it is working very well. Big plus is that it has 5 Ghz band support, so you can use that on newer devices (e.g. iPad, newer iPhones, etc.) instead of the old 2.4 Ghz which typically suffers from mentioned interference from neighbor WIFI routers and is faster. (Be sure to update firmware on the router. I personally use Merlin ASUS firmware).
Hope it helps.