Metagross' Steel/ Psychic typing means it has 2 weaknesses, while Tyranitar's Rock/ Dark typing means it has a critical x4 weakness to Fight, among other common weaknesses such as Grass and Water, having 6 weaknesses in total.
They both have sub-par speed, but Metagross is slightly faster at 71. Also, Metagross will serve as a physical wall, while Tyranitar is best used as a sweeper. Even though it has a high defence and sp. def, it has too many weaknesses to warrant usage as a wall.
You can always run an Iron Ball Trick set to deal with Pokemon like Skamory and Rotom (since Metagross has Earthquake).
Also, Tyranitar will always come with Sand Stream in Gen IV, meaning that your entire team will get buffeted/ affected by weather conditions.
Ultimately though, I think there are better Pokemon to function as walls (Slowbro/ Blissey), since Metagross has better things to do other than taking hits; it has no reliable method of recovery, and you'd be forfeiting its high level of attack if used it solely as a wall. If you want a Steel/ Psychic wall, go with Bronzong, since Levitate will mean it only has 1 weakness.
