I have Metalabs, love them. I didn't realise how much of a difference they made until I rode in 'normal' stirrups, walk and trot, for half an hour - my knees were very painful for the next couple of days! I occasionally trickle a little oil into the moveable parts as neither I nor the saddler could sort out the squeak in the saddle... because it was coming from the stirrup!
toyed with both and eventually plumped for Sprengers bow balance. Have you been able to try either type, I would recommend it if you can as they both good but i felt better with Sprengers