The answer depends on the use you make of the wireless connection. If you depend heavily on HD streaming you might want to get true 300 Mbps. If it is primarily internet traffic, Xvid/dvix video, audio streaming and some LAN file sending the 150 Mbps connection should do nicely. I guess you don't have a 300 Mbps internet access ISP

so you don't lose internet speed.
And to be honest, 300 Mbps is not the actual speed you get. You get between 80 and 120 Mbps in real life data transfer with full N (300 Mbps) due to overhead etc. Do not imagine you are going to 300mbit. That is still typical wireless over reach. It is still simplex ie it cannot transmit and receive at the same time.
To calculate the throughput in Megabytes: Divide and calculate bearing in mind that:
b= bit
B=Byte
8 bits= 1 byte
(

Just some maths exercises to get you through the holidays)