I was there a few weeks ago, so I don't know if my comments will be relevant for a stay in winter.

I personally wouldn't go back to St Julians, I just didn't like it. I'm sure there are nicer places on the island. Someone else might feel differently, of course.
In July/August it was absolutely packed with young people (17-25 years old), who were there to party. It was too crowded for my taste and not really interesting. Unlike Valletta, which is truly beautiful.

I'm not really sure how I feel about the hotel... It's nice enough, but way overpriced (235 EUR for a Privilege room with breakfast).

What bugged me the most is that there was no wardrobe in the room, just a few hangers, so I had to live out of my suitcase for 7 days. (Are wardrobes not a thing anymore? I've noticed this at other new hotels too.)

The other problem is that there is no way to close the door quietly, so you can hear door slamming in the hallway all the time (not good if you're a light sleeper). These two things (no wardrobe and door slamming) remind me too much of Ibis Styles hotels, except that this Mercure is much more expensive.
There was also always a terrible, sewer-like stench in the hallway on my floor (9).

Other than that, everything is OK. Comfortable bed. Nice staff. Lots of restaurants and shops nearby. Good breakfast. A small rooftop pool and bar. No restaurant and no food available at the hotel after 5 PM.

They are stingy with water: they only provide a free bottle of water in the room on the first day and that's it.
No minibar, but a small fridge where you can keep your stuff.
Good A/C.
Nice bathroom.
Clean.
At the moment there is construction work across the street (I think they're building a hotel, so I doubt they'll finish by winter), they start working really early (6:30 or something) and are quite loud. This is only relevant if you get a room facing Spinola Bay. I didn't mind it that much because I get up early anyway.

Loyalty treatment:
Welcome drinks for 2: soft drinks/wine/beer.
Welcome amenity: a bar of chocolate with sea salt (a local delicacy, apparently).
Late check-out at 1 PM wasn't a problem.
No upgrade, but I didn't expect to get upgraded anyway as it was high season and I was there for a week. They did, however, give me the best room in the Privilege category (room 911), with the nicest view from the balcony, both of Spinola Bay and the sea, and farthest from the elevator, which was really appreciated. Unfortunately, this room is also the closest to the construction work that I mentioned above.