You new router is likely following the rules and the other was cheating. The rule says if it detects a signal in a adjoining band....ie your neighbor it should switch to narrow channels. Ie 20mhz rather than 40mhz.
144 is a very telling number it means you are getting very strong signal and using 2x2 mimo on a 20mhz channel. If you were to allow it to use 40mhz it would get 300. There is a chart about halfway down that lets you see what the connect numbers map to.
http://en.wikipedia.org/wiki/IEEE_802.11n-2009
Most routers have the ability to turn the options off or force the router to use 40mhz. Be aware that if there is actually a very strong signal in a channel next to yours you make get better throughput using 20mhz channels. What you would see is it connect at 300m but it would get so many more errors due to the interference you would be better off connecting at 144 and getting fewer errors.
Tough to say you are just going to have to play around and see what really works best in your house.