They seem pretty similar to me. What's the difference between the tags? Should they be aliased or implied?

They are two different tags. Similar, but different. At least they should be, but it may be different to some mods.

Anyway, for weigh training, there are two types: mass and tone. Mass makes you build more muscle faster, which is where the muscle tag should come from. And then there's tone, which means less weight but more repititions, which causes you to slim down and burn fat rather than build muscle like crazy. This causes you to be slimmer and less bulky.

Jjiped's got it.

The muscular tag applies when a character looks "bulgy" or "lumpy."

e.g.
post #198078
post #91196

Toned describes characters that have visibly defined muscle structure, but the overall topology of their body is still relatively smooth and within "normal" proportions.

e.g.
post #195732
post #167609

Obviously there's a gradient between the two that characters can fall under: they can be toned but not muscular, muscular but not toned, both, or neither.

Side note: it's a bit difficult to find images on e621 of characters that are visibly muscular while lacking tone, but I'm pretty sure they're around here somewhere. Most artist that draw muscular characters just do away with fat entirely (see the two examples above,) and a lot of "fat" characters can be interpreted to have muscle mass underneath obfuscating layers of fat.

Edit: I've updated the wiki pages for the two tags.

Well I thought if you were toned you were muscular but ok ._.

KiwiPotato said:
Well I thought if you were toned you were muscular but ok ._.

Not necessarily. A person can have a whole lot of muscles and a considerable amount of fat; those people can be described as muscular but not toned. Think of sumo wrestlers: they're very fat, but they're also strong enough to lift each other off the ground (which you'd obviously need a lot of muscle mass for.)

As I said when it comes to drawings most of the time artists will automatically make muscular characters very toned.

Also, fat covers muscle. If a character does have a lot of fat and muscles, the fat will (usually) prevent you from seeing their muscles, even if they're fairly built. So on e621, in accordance with the tag what you see rule, it wouldn't get the muscles tag.

Tone and muscle mass actually have a much more complex definitions than simply "lack of fat" and "presence of muscles," respectively. But from a tagging standpoint they're just more useful that way.