That's not how it works:
- Segwit removes some data from transactions (signatures) and puts it elsewhere, this means that old nodes that haven't updated still process the transactions but they assume that the other updated nodes are honest. It effectively increases block size to 2MB, however, actual storage space requiremetns per block are more like 3MB or 4MB. Segwit however fixes transaction malleability, a long standing (though minor) issue/
- BTC unlimited allows miners to set their own block size and regularly re-set it as they wish. It works via consensus and is unlimited as the name suggests. It doesn't fix transaction malleability and allowing miners absolute freedom might be a bit premature.
Both have issues. BIP100 or Bitcoin ABC might be better solutions.