Heading to Malta in March, feedback please on Sliema vs. Valletta

We were in Malta last October. We stayed in St. Julian which was too far out. We went with others who planned the trip. We had to take taxis a lot.
Anyway, Sliema is lovely and a very easy, quick gondola like boat ride to Valletta. Sliema was more picturesque and not as crowded. Your hotel sounds fabulous so I wouldn’t worry.
Also, we took a fun, inexpensive walking tour with Malta themed tours. We will be in Malta again, this time off a cruise in early October, and have booked another tour with the same company. It’s pretty much a one-man show but definitely worth your time.

Actually, we didn't care much for either Sliema or St Juliens, and found them lacking in charm, but tastes vary.

When I started looking for places to stay in "downtown" Valletta, I did not find many options there - one grand old place from the Victorian era that looked v-e-r-y nice but was pretty expensive. I believe St Juliens has become the target for most destination tourism (as opposed to cruise ship tourism which is dominant), and that seems to be mostly budget fun-in-the-sun package tourism (It's also notorious as a party spot for youth). When the cruise ships are in (which is pretty much every day), parts of Valletta can get pretty swamped with crowds during the day.

We stayed just across the water in the "three cities" and loved these old, atmospheric, seaside neighborhoods. Note: they're not separate cities, they're just old neighborhoods on the opposite side of Valletta's harbor, a delightful 5 minute water shuttle away (buy a week-long pass, it's cheap, and the crossings are every 15 minutes or so - like a shuttle bus). And there aren't exactly "three" of them, they are not clearly defined and you could make the case there are two, three, four or five of them actually, depending on what criteria you use. And to add to the confusion, each the "three" cities that seem to have names have at least two completely different names, which are used interchangeably by locals. So stop expecting the naming of these places to make any sense.

I saw no obvious, large hotels in the Three Cities - none. There's a smattering of B&Bs, AirBnBs and an occasional little boutique hotel scattered around. So it may not be what you're looking for. But they're really cool old neighborhoods, on peninsulas, almost completely surrounded by water, but connected and walkable and full of old stone charm (and lots of vertical real estate and "streets" that are composed of steps, but all of Valletta is that way). Plenty of nice restaurants lining much of the waterfront, great streets to wander and explore. Very quiet at night, but some tourists do come to ogle the scenery during daytime.

If you're not 100% sure the Sliema hotel is your preference, I'd suggest exploring options in Three Cities. They're not super-touristy and the gems may be harder to find, but they're worth the effort.

While you're in Malta, don't overlook Gozo - it's awesome, less touristy and full of amazing things to see and do.

We were in Malta thrice, and tbh. felt Sliema not as charming as some other cities - the best place would be close to Manoel Island, which I feel your option might be around? I recall looking at one there. The recommendation of the Three Cities is a nice one.

Depending on what you are planning to do, also take connection via public transport into account. Connections to Mdina, Gozo or Marsaxlokk might be relevant.

March is off season, so prices should be rather reasonable.