The LowRider is cheaper and able to cut much larger work, and leaves most of your bench space available when you’re not using it, but you do have to make some flat plywood parts, and it hangs off both sides of the table. The MPCNC feels more like a traditional CNC machine and is easier to enclose, but I don’t think it’s actually any more accurate in practice. The MPCNC feels like it should be better for aluminium, but some LowRider owners have posted good results too.
Consider if you really will do large jobs - feed rates are typically under 10mm/s, so large jobs can take a long time, and you can’t leave a CNC router running unattended because it can catch fire in seconds.
There’s a list here of UK owners. Screwfix sell 25mm OD DETA conduit, which a lot of us have used. Stainless steel appears to be most common in 25.4mm - don’t assume you can get it in 25mm! You can get everything else from ebay, although a pre-flashed board from Ryan may be worth the postage costs, depending how comfortable you are with loading code onto arduinos.
Either machine can take a drag knife.