The main difference are that the 501 is not 900 lumens, and the Smudges XML is probably nearly 1000 lumens…
The Magicshine P7 bike torches (I have both a MS bike light and a P7 torch which is very similar to a 501) put out around 550-600 lumens (measured) with quite a floody beam but doesn’t have a huge throw/reach.
Smudges XML, quote 1000 and I imagine it is around this figure, as everyone else advertises them as 1200-1600 lumens if you check eBay. The beam from this is tighter & brighter but the reach/throw is much-much better.
I would simply suggest getting XML torches or smudges (or similar) units would be better than spending money on the old P7 LED’s. There not bad, but the XML’s are so much better for basically the same money.