Phono Preamp. With transformer or fully active

With one SUT you can't use all MC cartridges, such phono stage is not universal for all MC. It will work for some of them nicely, but if your cartridge impedance is very low (2 Ohm) or very high (40 Ohm) you can't use one SUT for both of them. 

Active phono stage with optional gain and optional loading is universal, one phono stage for all MC cartridges. 

SUTs are as diverse in their sound as phono stages are (maybe even more so), so it’s difficult to put all the stages with built-in SUTs into a bucket. And as mentioned by chakster, any given SUT is not nearly as adaptable to a wide range of cartridges as a typical active MC stage would be - so when picking a SUT you generally also have to select either a single target cartridge, or a narrow window of target cartridges for best results. It’s not going to work great if you have a low output MC, medium MC, and high MC in your collection. It’s not going to work great if you have a cartridge with high coil impedance for its output level, like a Benz Ruby. You must keep that in mind when choosing a phono stage with a SUT MC stage. Of course you can always do what I do - bypass the stage’s internal SUT and use outboard SUTs of your choice coupled to its MM mode.

SUTs are generally paired with tube MM stages to keep the tube rush noise down. Kind of the "best of both worlds" approach. But there’s no reason they can’t also work great with SS MM stages - it just seems less common probably for market factors more than anything else.

In my experience, two ends of the SUT sonic spectrum would be Bob’s Devices blue label CineMags with a warm, meaty, fleshy and seductive sound - and then Lundahl LL1931 amorphous cores with a crystalline, fast, detailed sound. Both of these have found their way into many phono stages. Personally I’ve found the CineMags to work better for me so far.

I chose my VAC Renaissance phono stage because its tube MM stage was by far the best I’ve heard, even though it uses Lundahl SUTs for MC mode. I just bypass them to favor my various outboard SUTs :)

As for transformer SUT vs. active MC stage (JFET), I have definitely preferred the sound of SUT so far. It’s generally more tactile, more dynamic, more alive sounding. JFET sounds more like a reproduction to me; I feel emotionally more distant to the music (slightly). But at this point I’ve heard only a few JFET stages and have a lot more experience with SUTs.

And one annoying thing with SUTs (especially outboard) is tracking down various hum / grounding issues. At least with built-in SUTs they usually get that right for you.

For DIYers, Jensen SUTs are also excellent, and Jensen offer technical advice on how to implement them, as do EMIA.  Bob's Devices, Lundahl, Jensen, EMIA, are only a very partial list of good companies that make SUTs. Having said that, I don't own a SUT; I do own 3 high gain phono stages that can accommodate a range of MC cartridges.

All these problems and limitations are out there and known, and that is why I have made the decision to not mess with them. Yes even knowing this means passing up the chance at discovering audio nirvana. The magic combination. Hats off to those who have. Me, I think just getting a record to play back properly is hard enough, without making it even more complicated than it has to be.

A lot of SUT's offer several settings, the Denon AU-320 has two, 3 ohm and 40 ohm. The Fidelity Research FRT-4 that Elliott just purchased has 4 settings so it covers a large range of carts.
A lot of hum and noise can be traced to poor planning, placing it next to an amp or other high power device along with running the cables close to power cables.Both Elliott and I were careful of our installation and are happy to report totally silent operation. A SUT that is set up carefully can be much quieter than an active phono stage. The quietest active MC phono stages usually cost big bucks as well.
BillWojo