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

Personal taste. If you can afford a dedicated phono pre, thats the way to go. If you are satisfied with MM cartridge and MM phono pre, and want to add a MC only, then the cheapest route (vs. a good phono pre with plenty adjustments)is a step up.

I use a very good separate phono stage [Blue Circle 707] AND a step up transformer [Bob's Devices]. Why? Because I like the way it sounds. See the review of Bob's in TEN AUDIO. I am a Blue Circle dealer but not connected with Bob's. This is a totally subjective area, there is no "best"; it is what YOU like.

Any argument that a stepup is theoretically inferior because it adds an extra device is simplistic at best, maybe specious. An active MC stage also adds an extra device. The question is, which device works best with any particular cartridge and the following (MM) gain stage?

The answer depends on the individual components.

FWIW, the active MC phono section in my $12K Doshi Alaap outplays my $800 Bent Audio Mu (copper version) stepups, despite fanatical tuning of the Mu's impedance for individual carts. What does this prove? Maybe not much, given the price difference. Still, though I haven't heard every stepup (not even the reportedly better silver Mu) no MC section of any type has ever outplayed the Alaap's IME.

One important fact: impedance matching with stepus is orders of magnitude more critical than with an active MC stage. True afficionados may use both primary side loading (for the cart) and secondary side (Ziebel) loading (to control transformer resonance). Getting all this right in a resolving system can take alot of trial and error work. Impedance setting with an active stage, while certainly audible, tends to be less critical.

Not to complicate an already very complicated matter, but if you have a good MM phono that you are happy with, you can also use one of several "head amps" or pre-preamps" that are available. These are stand alone active gain stages and would be used instead of an SUT. For a time, I used a ZYX (I think its called the CCP-1) and it was very good. There are supposedly 2 advantages of the active gain approach- the first one doug has elaborated on well. The second, actually a corollary of the first, is that an active gain stage (internal or external) is more likely to mate well with more than one LOMC, while its more likely that the SUT approach will require very elaborate tweeking and/or a different SUT for a different cart.

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