Grasen said:
and what is special about columbus slx
Everyone has their own opinions as to what makes a decent frameset....

I think SLX is similar to Reynolds 708 tubing, rifled pipes.
Personally I like 531c as I had a Mercian with that tubing and it was great but then it would be,,, being a Mercian.
A lot of it is down to the builder... and also you are more likely to find different tube sets being used for different types of bike. 501 can be found on all sorts of sports bikes but less so on really flash totally race orientated machines with swift handling; those will have 531c-653-753 etc
This might well mean the 501 framed bike will have slightly slower and more stable handling, not so much the tubeset but the use it has been put to.
but....
I have a time trial bike made from 501, it is more comfy than my old alloy one but I suspect it might have been better made with 531c as it would have been lighter and have a bit more zing if you know what I mean.
To put the cat amongst the pigeons here.... I have a Holdsworth with 531cs tubing.... excellent fast tourer type, nearly as good as my old Mercian..
Worth looking out for as it seems a decent compromise tube set that is built into practical bike frames, it was a tube set that was to bridge the gap between 531c racing tubing and 531st heavy touring so would have been used on club bikes and what would now be called an Audax bike.