MP150 or MP200

1+++ MP500:)  Very interesting system by the way. I like it!

You can change the stylus, not a cartridge
Get better nagaoka stylus

My system sounds ok . There is a video on youtube where they swap the stylus and it doesn"t seem to make much of a difference . Nagaoka states that in it of its cartridges the generator system is updated like better windings , purer metal and it seems that the cantilever assembly has been tuned for its particular generator . That would mean that one particular stylus and cantilever assembly will sound great with the cartridge its intended to but will degrade the sound if you changed it to another cartridge in its lineup . Another problem with Nagaoka is their pricing strategy.
 They went up in price some time ago and are expensive . I have the MP110 which cost me 132.00 . If I which to buy the next up in line , the MP150 I would have to shell out over 350.00 ! And the next one up would be the MP200 at almost 500.00 . And then there is the 300 and 500 which is 1grand !

You MP110 is entry level model with bonded elliptical diamond and aluminum cantilever.

Higher model like MP200 comes with NUDE (not bonded) diamond and Boron cantilever. One of the reason the price is much higher. 

Never use youtube as reference in cartridge comparison tests. 

As you said you want to stay with Nagaoka, so you could investigate which compatible stylus you could buy instead of a new cartridge. If you have cash for MP200 go for it. 

In my opinion you can easily find a better high compliance MM cartridge like SONY XL50 with Boron Pipe cantilever, but you want to stay with Nagaoka. 

$500-700 for a great MM cartridge is fair price. 

The MP-150 and the MP-200 share cartridge types.
Not saying it’s the way to go unless you are trying to keep spending increments low, but I bought the 150 for about $347 and a year or so later, picked up the JN-P200 (MP-200 replacement stylus) for $169 from a eBay Nagaoka dealer in Japan.
The MP-200 cartridge can be had for around $450-$500... so yes I paid a little more, but not much.
Note that the MP-500 Stylus is not compatible with the MP-150 or 200 cartridge, so the incremental upgrading stops with the 200. The JN-P200 stylus is not compatible with the MP-110 cartidge either.
I liked the MP-150 a lot and did notice an obvious improvement with JN-P200 stylus on my (budget system in this neighborhood) system. Better detail in percussion, brushes, stringed instrument fingerings, etc. Bass tightened up slightly... a little less "boom" and better detail as well.
My (phono) system: Technics SL-1610, Pro-ject Tube Box S2 phono stage, Emotiva pre-amp, Morrow interconnects and ZuAudio Libtec speaker cables feeding Magnepan 1.6 speakers.
I enjoyed making and hearing improvements and now I have a 1 year old, spare MP-150 stylus in case I do the unthinkable and damage the somewhat more delicate boron cantelever on the JN-P200 stylus.