I'd say that if you can't tell much from a level design / game design portfolio, then it's not a well designed portfolio (a nail in the coffin of a designer). A good design portfolio needs to communicate to the viewer what they are looking at, and why. It's not really so indistinct from the actual level / game design…
I'd say they are about equal, but level design might be a bit tougher to get, although like any other job in the industry it does rely on your portfolio quite alot. Level designer is also a good starting role if you want to move up to something like a game designer within the company in the future as well, so they are…
If you want to do level design then make sure to use your environments to do so. Just doing environment assets will never qualify you for doing level design. Level design is a part of game design, and does not require art per se. Level design is about composition, user flow, leading the player, placement of important…