which is better ? MC phono stage or MM+step-up ?

Question: which is better ? MC phono stage or MM+step-up ?
Answer: Yes.

Well said, Lewm!

I find that transformers tend to obscure detail and they limit bandwidth. So if you have enough gain, an MC high gain phono section might be better if properly designed.

The loading of the transformer is critical, else it can ring (distort), making things brighter and obscuring detail. All the following is assuming that the transformer is properly loaded:

If you do not have enough output from your cartridge, most phono sections will loose some bass impact. Where a transformer will normally do that too, in this case they will win you some back.

If you have too much noise, the noise will obscure low level detail. Where transformer will also loose detail, it also gets you gain without noise, and so wins you back some of that low level detail.

So- its not something that you can always state what will work in all cases. Usually the manufacturer of the phono section will know if the cartridge you have will need a transformer or not.

If there was a panacea, I would state it but as a manufacturer myself that is well-known for not liking transformers, I'll be the first to admit that they can be very beneficial in a variety of situations!

I used some MC Phono Stages with good results and I did listen to Transformer based units which offered more detail close to the real thing than anything else. I think, it depends on the designers abilities.

A low output moving coil cartridge is one half of a "team". The other half is a corresponding coil (matched to the coil in the moving coil (sic...) cartridge of course...) in a step-up transformer. These two units do form a team.
I would never view a LOMC as a "stand alone unit".
The matching and corresponding transformer must be precisely designed to match a certain source impedance and source inductance of the LOMC. Given precise match and technical inspired and executed design, we will look (...IMHO....) at the best possible solution in cartridge based playback.
This is true for LOMCs - medium output or high-output moving coils often do perform better with high-gain active stages. But a very low source impedance (below 6-8 ohms) will always long for a matching inductance more than a matching impedance.
In the end it all comes down - once again... - to precision, minute care and attention to detail.
There are no "universal approach" step-ups out there. There is always ONE matching inductance for a given LOMC.
You will find medium output MC performing to their very best with high gain active phono stages - and you will find LOMCs and ULOMC (ultra low output moving coil) performing their very best with precisely matched step-ups of the highest caliber.
Enjoy the search and the long path of try and error.....