Ive used there M/S wire almost exclusively for years, never had any problems with it, has always been pretty consistant and well wound, Al`s probably right about it being Chinese though.
Havent tried there Ali or Stainless wire, always kept to the bigger names for that.
I suppose the best wire wouln't be any good in a bad machine. The way to be certain would be to listen to people's recommendations, buy from a welding supplies specialist and perhaps buy a small reel to try.
Can't comment on Screwfix wire as i have never used it, but using better quality brands from known suppliers is deffo worth the extra money - wire feed is much better, less rubbish on the surface of the welds, and little chance of corrosion on the wire before it is used!
We use pallets full of Nexus stuff. Seems good gear for the right price. Very few complaints.
Also can't comment on Screwfix wire, but I've had issues before running cheap wire on spray transfer - major bubbling and porosity.
Technic Al persuaded me that Lincoln SupraMig should be my wire of choice. It's good stuff, but costs more than Nexus etc. As I'm a fairly low volume user the extra expense isn't important.
I imagine that for short-circuiting, the majority of wires, cheap or dear, will be fine.
The quality of the winding can also be a factor with cheap wire. I bought a 5kg reel of SIP stuff from Halfrauds and it caused no end of feed problems once I got down to the last 1/4. New reel on (same stuff as it was Sunday ) and I ran it all out with no issues at all.