Antlionerd

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

Ferrothorn:

Hp:74
attack: 94
defence:131
sp attack: 54
sp defence: 116
speed: 20

while registeel is......

Hp 80
attack 75
defence 150
sp attack 75
sp defence 150
speed 50

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.

@VibravaRules

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.