Apologies if I go over anything you already know =). Also, if you don't already have one, I recommend Acrylic WiFi Home (free) as a good home network scanner.
But to the questions...
My notebooks both have a Wireless-N 2x2 adapters. The maximum link speed they can get on my primary router 150 Mbps as against that of 300 Mbps on my AP (Mind you both are only on Wireless N and not on Wireless AC). Why is there a difference in spite of both being N300 routers?
So we are all clear on this, because a device has a theoretical maximum speed, it does not necessarily mean that device is always attempting to operate at that maximum speed.
Likewise, having a 2x2 adapter does not guarantee ~300 Mbps on Wireless N. For that you need two spacial streams (2x2), a 40 MHz (effective) channel bandwidth, and a short guard interval (short GI - about 400ns). Bandwidth is the key here. 2x2 and a short GI with a 20 MHz bandwidth channel will only net you ~144 Mbps or less.
In general, possible reasons for the speed disparity between these devices include:
Settings - Wireless devices frequently have settings which allow the theoretical maximum transmission speeds to be set (e.g. ~150 or ~300 Mbps for Wireless N). One or more of your devices may not have this set correctly.
Interference/Attenuation - These can cause aggressive throttling and packet loss (both bad for speed). Packet loss comes from direct signal interference or signal attenuation (weakening), while throttling is due to frequency congestion and the fact that wireless connections have no packet collision detection (so devices must be extra careful when sending data in "busy" wireless environments).
Interference sources include:
Spurious interference from other nearby electronics (e.g. cordless phones, other WiFi networks, your microwave) (packet loss or erroneous throttling).
Devices waiting for other (genuine) devices. This could include having too many wireless devices hooked up to one router or devices operating on the same WiFi channel(s) (throttling).
Attenuation sources include general distance between devices and physical structures (windows, walls, doors, floors, etc.). Low power output can also have a negative impact (this can sometime be overcome in device settings).
Channel Bonding - The "40 MHz" channel bandwidth needed for ~300 Mbps on Wireless N is frequently achieved via channel bonding (using two 20MHz bandwidth channels at the same time). If one of those channels is unreliable (due to interference, etc.), ~300 Mbps cannot be achieved (most devices fall back to using a single 20 MHz channel).
Hardware - Devices sometimes just simply do not support ~300 Mbps, especially when operating on 2.4 GHz. This can be due to age of the device, poor quality components, regulatory adherence, over-cautious programming or other interoperability issues.
[My] [o]ther question is about my mobiles devices which run on both N and AC. For Wireless AC, the devices catch the complete link speed ie. 433 Mbps however for Wireless N, these devices get only a link speed of 72 - 75 Mbps which is [nearly] 1/4th of the N300 speed. Why does such [a] difference [occur] in link speed? Does this have anything to do with lowest device speed or similar sort of phenomenon?
The only time I ever personally see speeds that low is with extreme signal attenuation (weak signals). My guess would be low-power mobile devices vs. long distance with lots of obstacles (and yes, Wireless AC will reach places Wireless N will not).