So, I have an Elys installed today, and I was thinking about the ND5, but after reading a lot of people saying I should go for the ND7, I'm not sure. Now you're suggesting an Audio Technica. Which one would be equivalent to an ND7, and wouldn't have much trouble with the installation? Or since I have an Elys, would it be silly to make this switch? Sorry for so many questions.
I haven't been able to find any measurements of the frequency response of the ND5 or ND7, but the Elys has a drop followed by a small spike around 10-14 kHz.
I used to have a Rega Exact, which is somewhat similar to an Elys, but less muddy, and the spike is bigger, giving it a clearer while still relaxed sound.
I then switched to an Audio Technica AT33PTG/II, which is a favourite of many. When I first listened to it I was smitten right away.
It's slightly more expensive than an ND7, and it's also an MC cartridge, but it is known for having a relatively accurate frequency response with a light dip in the lower treble and a relatively small spike around 10-14 kHz, and a very smooth but detailed sound with low distortion.
I also have an Audio Technica VM750SH, which I like with many older records. Its frequency response is similar to the AT33PTG/II but with a bigger spike, which gives this perception of more "air".
I have also tried the OC9 and ART9 from Audio Technica, and I definitely preferred the AT33PTG/II. The OC9 was harsher and brighter, which it's also known for. The ART9 just didn't seem to track as well as the AT33PTG/, so I heard more noise between tracks. However, this can also be because of how I set it up though, or because the ART9 doesn't have the light dip in the lower treble, so then the noise in that region is more audible.
The AH-95 is a cheaper model, but as far as I know it performs very well for its price. The one called SH at the end of the model name has a shibata stylus and also a bigger spike in the high frequencies, thus giving more "air". The one called ML has a microline stylus.
The flattest frequency response in any cartridge I have tried was the Nagaoka MP-500, and I have seen others find similar results. As far as I know it also has low distortion.
It's an MM and not hyper expensive, although not a cheap MM.
Their cheaper models have less flat frequency responses.
I liked the MP-500 very much, but for some reason I wanted a bigger spike in the high frequencies to have a bit more "air", so instead I chose a Goldring Elite, but in the end switched to the AT33PTG/II.
If your goal is the flattest frequency response possible, chose the MP-500.
As for Ortofon, I haven't tried the 2M series myself, but I've heard recordings, and to my ears they sound harsh and overly bright. I've tried Ortofon's Quintet Black though. It seemed okay, but I wasn't smitten by it, which I know is a very subjective assesment.
As you can see, a lot of subjective preference goes into choosing a cartridge.
Although I can't say anything about the ND5 and ND7, I think you can find a good cartridge based on price and what kind of specific sound you're looking for: Completely neutral (MP-500), smooth yet detailed (AT33PTG/II or VM-95 (perhaps the ML version)), darker and less harsh (Grado, Rega Exact, etc.), or very detailed and a bit bright (various Ortofon).