At the start, I'd say I watched anime first then read the manga later. Now that I'm more into manga, and I've read manga that don't have anime yet, it's probably the other way around. If an anime adaptation is good, you can totally enjoy it despite knowing what events take place.
If someone believes that reading manga is better than watching anime, their reasoning will be a combination of the best perks of manga and the worst aspects of anime, or a combination thereof. For example, while some anime series manage to tell the complete story of the original manga, most don't.
Many modern anime series have excellent animation techniques, such as the popular Attack on Titan and Vinland Saga. Others have just mediocre or notoriously bad animation, such as The Seven Deadly Sins, which has inspired some unflattering nicknames such as "the seven deadly frames."
Anime streaming platforms such as Crunchyroll and Funimation are excellent sources of anime, along with Netflix and Hulu. Still, some series are never streamed because certain platforms simply don't have the rights to them. Some series, such as Wotakoi and Grand Blue Dreaming, are barely streamed anywhere at all in the West.
In general, people read words much faster than they can speak, which means an anime show or TV show will deliver its content more slowly than any comic book, novel, or manga volume. A sentence that can be read in two seconds might take eight or more to speak aloud in anime.
A particular downside to being an anime fans is the fact that many solid manga series never got an anime at all, so an anime-only fan is locked out of these stories entirely. Some anime fans won't mind, but others will realize just how much they're missing out on.
Granted, some manga series do have fairly simple art that is easy to adapt into a lovely anime series, and a few manga series are actually improved this way. But the reverse is more common, and that works in any manga fan's favor. Some art simply cannot be animated.
Not everyone has the budget or the room at home to collect dozens or hundreds of manga volumes, but it is a wonderful hobby to have for those who do. Some people love to collect physical items, and manga fans can buy a few bookshelves and stock them up with every volume of their favorite series.