How many people are connected to the same network? That 300MB/s you get, that's shared equally between users.
How far are you away from the wireless access point/router?
Also, just because your other adapter is capable of 150MB/s, it doesn't mean the new one is. Not all wireless adapters are born equally. I can't tell you how many there are out there that while they theoretically can receive 300MB/s, have such terrible signal dropoffs that in a realistic setting, they'll be receiving barely any of the advertised speed. You know, most of those 300MB/s advertisements are based on the specification they're made to during manufacturing and most of them aren't truly tested in typical situations before being shipped out. At best, they might test it from 10 metres with nothing in between the adapter and the router.
Some companies do test, but since there's no standard on speed from a specific distance, they're not forced to test it in those situations. It's one reason you'll only ever see distance and speed, and not a combination such as "you will get X speed at Y distance".
Also, you need to note that upon initialising any network connection or activity, there's an initial surge in speed. Just as with file copying, the speed starts off real high, but soon settles down to what it's actually getting. In reality, you're never actually getting that initial speed, it's just the software calculating your real speed. Some software starts off at 0 and calculated up, other starts off at a high value and calculates down.
Judging from Netgear's forum, it's not uncommon that people have reported issues with their products not reaching advertised speeds. Although it's not the same product as yours,
here's a thread from 2010 indicating speed issues with a similar product.
It is possible there's something we're missing, but it's not as black and white as "my other dongle can manage this speed so why can't this one".