BLUFF (bottom line up front): If you like the sound of the M-Line get a matching cab. The M-Line comes in 4 or 8 ohms, so hopefully you have the 8 ohm version.
If you want a different type of sound, consider the NL (IMHO try before you buy).
I don't have recent experience with either cab. But as @tombowlus indicated it really comes down to whether you want a voiced cab (M-Line) or a more neutral cab (NL).
I will quibble with Tom's points about volume though, as power handling is only part of the max SPL equation. The Wizzy M-line is rated for 200W and max SPL of 126dB at 1m. It's true that the NL112 is rated for more than double the power at 500W, but the max SPL only goes up by 1dB to 127dB.
Sensitivity is the other factor you need to consider to understand why it works this way. EA does not appear to post Sensitivity ratings for these cabs, but it's not hard to estimate them using this webpage:
Watts to dBm conversion calculator (rapidtables.com)
You can't use dBm directly, but you can use the calculator to determine the decibel change that occurs when you vary the power level. For example 1W is 30dB and 200W is ~53dBm. Subtract the difference and drop the "m" and you get the dB change that occurs when you change to power from 1W to 200W.
The result of increasing power from 1W to 200W is a 23dB change. Subtract the dB change from Max SPL and you get the 1W sensitivity rating. So for the M-Line we get the following Sensitivity rating. 126-23=
103dB 1W/1m
The result of increasing the power from 1W to 500 is a 27dB change. So for the NL we get the following Sensitivity rating: 127-27=
100dB 1W/1m
So the M-line is 3dB more efficient than the NL.
Keep in mind you need to double the power to get a 3dB change. So if you work through these concepts, you will realize that the NL112 will require 400W to play as loud as the M-Line plays with 200W. In other words the NL112 needs 400W to make 126dB, while the M-Line only needs 200W.
My advice is to choose the cab that sounds best to you and order two matching cabs in 8 ohms. Here's why: Two cabs placed close together couple acoustically. The coupling increases how efficiently the cabs translate motion into sound.
The results are a bit confusing:
Essentially two cabs together will sound 6dB louder than the SPL produce by one of the cabs.
Assume you have an amp that makes 200W at 8 ohms and 400W at 4 ohms and we are using the M-Line. You when you use one cab at 200W it makes a Max SPL of 126dB. When you add the second cab, the amps power doubles to 400W. The available power is then split in two, so each cab gets half. So both cabs are receiving 200W. Therefore you have two M-Lines that each get 200W and make 126dB. For the frequency range in which driver spacing is within about 1/4 wavelength you get near perfect acoustic coupling, which produces an additional 6dB. So the max SPL of two M-Lines sharing 400W is 126+6=132dB.
The acoustic coupling only happens at the lowest frequencies. So running two cabs tends to make the lows fuller and thicker.
Many amps do not double power when the impedance drops from 8 to 4 ohms. With solid state amps the power typically goes up, but only by 60% or so. Consider if the amp is rated for 200W at 8 ohms and 320W at 4 ohms.
Now each cab shares 320W. 320/2=160W. Now we need to your our online calculator again to find the dB change that occurs when power varies from 1-160W. The calculator says this results in a 22dB change. Add this to the sensitivity rating. 103+22=125dB. So one cab will make 125dB with 160W. If you add a matching making the same SPL you 125+6=131dB.
So in this scenario one cab makes 126dB with 200W, and two cabs sharing 360W make 131dB.
There is one more scenario to consider. Tube amps use a transformer to match their output impedance to the impedance of the loud. So as long as the amp sees the expected impedance, it will make max power. For example if your 200W tube amp has 4 ohm and 8 ohm output taps, it will make 200W at both 4 ohms and 8 ohms.
When power is held constant, you only get +3dB from acoustic coupling. The issue is when you add the second cab, available power is split evenly between them, so each cab will only get 100W. When you cut the power in half you get a -3dB change. In other words, if one cab makes 126dB with 200W, two cabs sharing 200W will each make 123dB.
You can confirm this with the calculator if you like. The dB change from 1-100W is 20. Add to the sensitivity rating: 103+20=123.
Now since you are running two cabs, add in the +6dB from acoustic coupling: 123+6=129dB.
So in this scenario, one cab makes 126dB with 200W, and two cabs sharing 200W make 129dB. As you see, when power is held constant, you get +3dB from acoustic coupling when you add the second cab.
