MP150 or MP200

Depends on the rest of your system. If it’s low budget, you may only notice a small improvement. Rather than making small incremental changes, save longer and upgrade your table. Then add the MP500.

Project 1 expression2 with modifications and speed control . elekit tube preamp upgraded and modified using Jan 12AT7 with SS phono stage connected to Mccormack DNA125 amp 
 All interconnects , speaker cable and and power cable are JPS Labs superconductor connected to Richard Gray power station . Speaker are Magnepan MG12 with external upgraded crossovers . Digital duties are carried by a Opera Audio Turandot CD Player 

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 !