Segwit allows for optimization which gives some scaling while keeping the basic status quo and network decentralized.
BU allows for direct increase of on-chain scaling which give more power to miners and centralizes the network.
This is the official propaganda. Segwit calls for centralisation through a banking layer on top of bitcoin, the lightning network. BU implements dynamic block sizes.
The totally ridiculous argument that BU would centralize, is that non-mining nodes, which have nothing to say in bitcoin, contrary to all the propaganda that tells the opposite, would need somewhat better network connections and bigger hard disks. But bitcoin is a *proof of work* consensus system, not a *count of node* consensus system, so no matter if there are many, or few, non mining nodes, the only true consensus mechanism in bitcoin is delivered by miners: proof of work. And that is inevitably centralizing.
The people making decisions in bitcoin are the mining pools: they decide what blocks to build, according to what protocol, on which other blocks. They decide of the choice of transactions and on all the rest. Strictly speaking, if you have 51% of the hash power, you can decide all you want in bitcoin. Well, here they are:
https://blockchain.info/poolsCount the number of deciders that need to collude to reach 51%. THIS is bitcoin's centralisation. Not the poor guys installing nodes in their basement that do not mine. These are nothing but proxy servers to the chain that these people decide to make.
So at this moment, bitcoin is decentralized over 5 entities for a small majority. 9 entities if you need 75% majority. Put 5 people in a room, and you decide with small margin. Put 9 people in a room, and they decide with 75% majority.
That's bitcoin's centralization.
I know the propaganda tells you otherwise, nodes blah blah blah. It ain't so. Bitcoin is an oligarchy of less than 10 people. But they do their jobs correctly. Because bitcoin is a proof-of-work consensus system, and proof of work is in the hands of less than 10 people.
The miners' hardware aren't in their hands, true. But the miners' hardware is not deciding upon which blocks to build, according to what protocol. They only sell hashes to the pools.