RF noise on different WiFi adapters
First I would just like to clarify few things that could influence this and I think you are misunderstanding a few concepts.

1) Receiver efficiency

a) Due to the antenna: Just because your two antennas are in exactly the same location, does not mean they will receive the same amount of power. Antennas are all directional - it can be proven that a truly omnidirectional antenna is impossible. What this means is that every antenna is more sensitive in certain directions than it is in others. You can't know how every antenna was "oriented" and this could make a big impact. 

To test this, what you can do is repeat the measurements with just one of the devices, but every time you do, rotate the device a little - both horizontally and vertically. As a result, you should find that the received power goes up and down depending on the orientation. If the different devices were not oriented the same way you will get different results - even if they were exactly identical in every other way.

In addition, not all antennas are equally efficient. Some antennas do a better job of "absorbing" the electro-magnetic wave and turning it into an electrical signal. A small antenna in a dongle is likely to be less efficient since it has to work in a smaller space and on a PCB, versus the "dedicated" antennas often found in laptops. This could add a few more dB loss, resulting in a weaker received signal.

b) Due to the receiver itself: Do you know where the power of the receiver is measured? Is it right at the antenna? Or perhaps after a first filter to isolate the wifi channel? Depending on how much there is between the antenna and the point of measurement, you will find that you might get different results. The manufacturer might have compensated for this, but you don't know. This could (and likely is) another source of error: there might be losses between the antenna and the actual power measurement, causing one to read lower power levels than the other.

2) Signal strength is different depending on location

You say that you measured in the same place, but how much difference is there? Even a few tens of centimeters could have a large impact because of the complex behavior of reflections you get with WiFi indoors. This could result in there actually being a significant difference in signal strength, even though they are measured in very close proximity. 
This video pretty much discribes what I am trying to say here: 
High Res Wifi Signal Mapping by user CNLohr on youtube

3) Errors in the actual measurement

How accurate are your measurement devices actually? Making an accurate system to measure power at these frequencies is not easy. Your WiFi system won't measure to single-digit accuracy. Especially if it is a cheaper dongle. So it could be that your dongle is just poorly calibrated, and this introduces an additional error.

Now to get to the noise

You have assumed that the received power could be split up into three groups: The desired signal, interference, and noise from internal circuits. 

However, did not take into account the external background noise. Even if you are in a area where there is nobody transmitting - nothing at all, you will receive noise. In fact, if I just take a coaxial cable which has been terminated, and I connect that up to the receiver - it will see noise. This is thermal noise, and is due the thermal radiation of everything around us. Unless you are in a busy area, most of your noise will be of this type. This is always there, and you can't get around it. This is why the antennas of NASA telescopes use amplifiers cooled to very low temperatures, sometimes just a few tens of Kelvin. 

For a WiFi channel (20 MHz bandwidth) at room temperature I believe this noise is around -110 dBm or so. 

You ask 


  (...) can I assume that the signal to noise ratio for both adapters will be the comparable, if only dominating noise from group (2) is considered?


The answer is that nobody can tell you. Because of all the issues above, you can't know for sure what caused you to measure different results. If the interference noise is hitting one antenna in a direction where it is sensitive, it will see this signal as being "more powerfull" than the second antenna which is less sensitive in that direction. In addition, this baby monitor will not be constantly transmitting - the -60 dBm value you are getting is a bit meaningless by itself, without more information about how these devices measure them (is this peak received noise power? Is this average received noise power?). Hence, it is a bit difficult to talk about the "SNR" in this way.
A final factor is the way they are all transmitting. If the baby monitor is connected to the same network, the WiFi protocol will actually try to avoid them talking at the same time. This illustrates the earlier issue of not knowing exactly how the power is measured and that "-60 dBm" figure is created - since each device will try and avoid transmitting at the same time as another device, they will not be a real noise signal you can treat like that.