Alloy ones are lighter, and have the same S4 spacing. Sorta no-brainer if your changing them. Cost me £60 for the pair of alloy uprights. I also picked up a set of S4 spaced 312mm carriers for 30quid, much cheaper than the TT ones go for, and 312's are the biggest disks that will fit under my stock wheels.
The S4 has its original S4 uprights on it, the alloy ones are for my car. Its needing wheel bearings, front arms etc, so i decided to get the alloy uprights and fit them out with new bearings ready just to swap over.
The inserts are for the bottom balljoints. Instead of the balljoint bearing directly against the alloy, there is a steel insert pressed into the upright, and the taper seat is machined into the steel. Unfortunately when people get hamfisted with hammers trying to get the arms off, often the insert will come out the upright, rather than the arm coming out the insert. Audi dont sell the inserts on their own, so i was glad he still had the arms and posted me them!
My cars going to be getting some major work soon, lots of little jobs have been building up but they're all interrelated. Front CV's are shot due to me driving around with a split boot last year. Boots fixed but the joint now clicking badly. Front brake disks and pads need done. I have a noisy wheel bearing somewhere, so they need done. Front arms are starting to knock a bit so those need replaced.
I've got a 6 speed S4 box sitting that i've been wanting to fit, so i figure that given i need CV's anyway, and the subframe needs to come out to fit it, i might as well fit the box, with the S4 shafts, and do the arms and brakes at the same time. Unfortunately, i've now got most of the bits together just as the weathers starting to turn to ****.