Lately I've been doing the trend of using passwords that are sentences
We call them passphrases ;-)
at least on sites that have a reasonable maximum character length and allow spaces
All websites should allow that. Contact an admin and refuse to create an account if it doesn't. They should store the passwords in a fixed-length format (the technique is called "hashing") which makes that, for them, there is no difference between a 200 word long passphrase and a 3 character password. Both would be converted to a fixed-length piece of data by hashing. The fact that certain characters are disallowed indicates that it does matter what password you enter and, though it might still be fine, it stinks of bad password storage techniques.
the most basic variation is 'This is my password.', or 'This is my <insert website here> password.'
Okay, but please do remember that those are just that: "basic variations". They are not secure and should be used for useless, throwaway accounts only (like a temporary one to view an article, or to download something from Oracle, or you name it).
Passphrases work best when they are composed of random words. We, as humans, are bad at randomness and are unable to pick a random item from our entire vocabulary. It would be better to take a paper dictionary and roll dice to find a random page and a random word on that page.
If you don't have a dictionary handy, trying to make up random phrases with illogical constructions (e.g. "potato red red twitter mind") is a reasonable substitute.
You should be distrustful of online services offering to generate passwords for you. Even if they don't, they could technically store the password that it generated for you, or pick the "random" password from a list of only a few hundred possibilities (and yes, a few hundred is very few because computers will run through all those possibilities in a very short time).
It seemed rather silly to memorize a group of random words instead of simply rather constructing a sentence, which is way easier on muscle memory. Is this easier to predict?
It seems I've already answered this, but to reiterate: Yes, sentences are easier to predict than random words. The point of a password or passphrase is to be something only you know. Using phrases that make sense to us limits the number of possibilities, making it easier to guess. It's simply more probable that the word "red" follows after "the" and not after "potato" ("the red something" vs "potato the").