Hey there! The sloop and galleon are pretty different ships, and both offer unique upsides and downsides. I'll try to list off as many as I can here, and how they can be used.
The largest balance point between these ships is crew size. It's gonna seem like I'm just listing off pros to the sloop here instead of qualities of both ships, but you have to remember that the sloop will only have half the people to work with right now, making these things harder to accomplish, but more rewarding.
One of the most easily noticeable differences is the firepower - while a sloop has 1 cannon on each side, the galleon has 4 on each. This gives the firepower advantage to the galleon, as long as you have more than one person available to use the cannons.
This takes me to my next point, which is boarding. One of the most common strategies in combat is getting on an enemies boat to kill them, drop their anchor, disrupt them, or just prevent them patching their ship. The sloop only has 2 crew at most, making boarding much more dangerous by leaving your boat more open to attack. The galleon definitely has the advantage here, as it has 4 crewmates, making it much easier to sacrifice one in an attempt to board a ship. However, it's also worth noting that the position of the ladders on a sloop lets the helmsman easily watch for boarders and take care of them before they can get on the boat, unlike on the galleon where this job would have to be left to someone else (and is often left undone).
Next, we have the sails. Since the sloop is made for a player to be able to sail alone, it only has one sail. The sloops sail adjusts faster than the galleons, making it easier to catch the wind when turning or in combat. But, a galleon can move faster with the wind, as it has 3 sails.
After that is the helm. The helm of the galleon takes two full rotations either way to turn as quickly as possible, and even then cannot turn as quickly as the sloop with its helm fully turned (which takes only one rotation). Also, with the position and size of the sail, you can see in front of you much more easily, while on a galleon you'll often have to drop the helm and move to see where you're going, or get help from a crewmate. The same goes for the map, as on a galleon the map is not visible from the deck the helm is on, but on a sloop it's visible from just behind the helm.
After that, we have the anchor. The sloops anchor raises about 4 times as fast with one person, and twice as fast with a full crew (2 for a sloop, 4 for a galleon). This, plus the turn radius, allows for faster and more effective anchor turns.
On top of that, the sloops relative size to the galleon makes it a much harder target in combat, and having only one floor below deck limits the number of holes in the ship enough that just one person can patch it (there are actually few enough holes possible in a sloop that if two people are bailing water from it with every possible hole, they can keep it afloat). It's also much easier and faster to patch and bail, since you only have to patch one floor on the sloop and can bail from the same floor.
All together, the sloop wins its fights not through sheer firepower, but by outsmarting and outplaying the other ship's crew through its maneuverability and its ability to easily stay afloat and void of boarders.
Personally, I prefer to sail on sloops, because they take less manpower, and I have to rely less on my crew when fighting in one. Also, because as long as you know how to sail one, it's very unlikely you'll ever sink. I've gotten into well over 100 fights on a sloop (mostly with a second crewmate) since I started playing this game. I've been sunk twice, and sunk almost every other ship I've come across (there have been 2 times where we ran each other out of resources and left at a stalemate). Whether it's a galleon or a sloop, the fights tend to be about the same difficulty, and much more dependent on the other crew than the type of ship.
TL;DR- Neither ship is inherently stronger than the other, and both have good chances of winning a fight when playing to their strengths. A sloop has more of a hit and run playstyle, while the galleon hits as hard and as often as possible. Both of these have pros and cons (as described in my post), and really depend more on the players understanding of the ship than the ship itself.
