Is sous vide better for lean meat or fatty meat?

Better cannot be answered as its opinion based, the question is do you like fat? What you have to remember with sous vide is that nothing in that sealed packet is going anywhere. If you roast or grill a piece of meat fat will drip off, if you sous vide it will all remain in the packet. Some of the fat will come out into the liquid surrounding it, but it will still be bathing in it, so a fatty piece of meat will give you a fattier result than other methods.

If you like fat that's a good thing, if you don't it isn't.

When cooking a steak sous vide to medium rare you're cooking at a fairly constant temperature around 129-135 Fahrenheit.

This temperature isn't enough to render fat quickly so a lot of people might complain about 'rubbery' fat, especially if it's a cut with a nice chunk of fat on the side. You also won't get a good sear with sous vide only. So I'd recommend either searing before or after the sous vide for a good crust which also helps render the fat.

Overall it's about preference and experimenting. The best part about cooking is trying new things and seeing whether you like it or not. Maybe fatty sous vide is perfect for you without a crust. Maybe try different setups and see what you like the most.

Lean meat is harder to cook correctly. If overcooked, it gets tough and dry, whereas fatty meat can take more abuse. I can't count how many overcooked porterhouses I've had where the strip side was decent but the tenderloin was dry. That won't happen with sous vide, and it's much harder to overcook something underwater compared to using fire. Sous vide will let you turn out well-cooked lean cuts with little effort and not much skill other than pre-plate browning.

On the other hand fatty meats tend to taste better and are cheaper. These often have more connective tissue as well, making them consuming to prepare and less marketable than other cuts. Poor Italian immigrants would slow cook cheap beef cuts in broth to produce Italian Beef, which is very tender. Sous vide is similar to that process. It will liberate all the fat in the meat, where it can be discarded or incorporated into the meal as desired.

In short, for average cooks compared to other methods, sous vide will let you cook more consistent lean cuts than otherwise and cook fatty cuts with less effort like trimming, monitoring flare-ups, and flipping while cooking.

A big problem with traditional fatty meats such as brisket, pork butt, or such is that Sous Vide temperatures don't get the meat hot enough to render the fat. Lean meats are hard to get tender because of the lack of fat, so Sous Vide is better for lean meat.

I would argue that for things like Brisket and Pork Butt that low and slow without using Sous Vide will always give better results.

For steaks, you can sear after Sous Vide, but just trim the excess fat because it will be easy to get the steak too done if you try to let it render all the fat.

The answer is both.

The beauty of sous-vide is that the precision temperature control gives you more flexibility with time. This enables you to serve meat that is still pink (because most proteins are still intact) but with the fat properly rendered and the connective tissue broken down.

One of the finest things you can do with a chuck or a brisket is cook it sous-vide for 2-3 days at 58°C, it is sooo juicy and tasty.

Conversely, if you do that with lean meat it will turn to mush due to the lack of connective tissue. These cuts should only be cooked for 2-3 hours at most, less for fish. The flexibility here is just to cook it through without overcooking.

Stay above 55°C for food safety reasons. There are some pathogens that won't die at 55°C however none are active. 55°C for two hours is also the recommended time and temperature to pasteurize an egg in Modernist Cuisine and elsewhere.