I need a mixed wall for my sand team, whos better to use?
Either one will work, really. Ferrothorn has Leech Seed and higher attack power, while Registeel has slightly higher defenses. Not to mention the type difference.
On a sand team, I'm honestly not sure. But I've used Ferrothorn, and I can honestly say that it's an amazing wall, a superior lead, and one of my best team members.
I went onto bulbapedia, and these are both registeels and ferrothorns base stats
sp attack: 54
sp defence: 116
while registeel is......
sp attack 75
sp defence 150
But seeing as your looking for a wall i would go with Ferrothorn. im not a compeditive player, so take the people above advice before mine.
ferrothorn anyday. it has a better movepool, better typing, has a semi-reliable recovery in form of leaach seed, can set up entry hazards, can do some damage with gyro ball and is a great anti-rain pokemon.