Neither. You don't go around making walls just for the fun of it. What you need to do is check your current team. Which type of walls does it need, which mons syngergize with the rest and more importantly, can you afford to spare two slots for your walls?
I've said this in an answer to a mixed attacker question but I'm too lazy to find it so I'll just re type what I mentioned. Whenever you're building a team, always check what it needs and find mons that could plug that hole. If there is a mixed wall that synergize with the rest of you're team then go for it. If you can afford to spare two slots for your walls and you found two mons that work well with the others, then use it
Bottomline is, always check first with the rest of the team and see what you can work with