I have a 7 month old chocolate lab puppy and I've heard that crate training is essential while he's still a puppy, my question is: We have a plastic pen that we put the puppy in, it's different from a crate because he can't get in it by himself and the space is still relatively the same because we can change the pen from being smaller or larger. Is this still efficient at training the puppy or is a crate better to use? I'm asking because we've had the puppy for almost a week now and if a crate is better, I want to purchase a crate right away and start training him with that instead. Any answers will be helpful! THANKS!

I don't quite understand what you mean by plastic pen?
Like a child's crib?

It is always better to have a crate that is fully enclosed so the puppy doesn't try to jump and hurt itself trying to get out.

I had a friend who kept her shih tzu in a puppy pen and it tried to jump out and got her mid section stuck... thousands of dollars worth of surgery later she still didn't make it.

The crate training gets them used to being in the same environemt their whole life...
If you have a pen now then switch to a crate later it will confuse the dog when it is older.

Always keep them very secure when you need to go out and leave plenty of water and toys.

I would probably recommend buying a crate now that will be large enough for an adult lab... it will not only save you buying two crates but as a puppy he will want to play in it while you aren't there and you can separate the back half for his bed and put newpaper in the front half for his toilet.