Well, I have or have had all of the above
I think the metal expens are the easiest transport and still give enough floor space for 2 small dogs. Be sure to get one tall enough, so they don't climb out. The metal pens fold up flat.
The plastic pen gives more floor space but it is bulkier, and not as easy to take up and down. It's also heavier to carry.
The doggy playpen doesn't give much space and it says on there not to leave dogs alone in it.
Like I said, for your needs (most portable and roomy) I vote for metal expen.
good luck