Generally - RHS (Rolled Hollow Section) is considered as a structural steel and as such will be less less strict tolerances then ERW (Electric Resistance Welded) or CDS (Cold Drawn Seamless). RHS will normally come in the 'black' form straight out of the mill whereas the others are likely to have been cold rolled and/or pickled to get rid of the scale. For our applications, the existance of the weld does not really make any difference. Typically ERW is so good nowadays that if you cut the tube and look at the weld under a microscope it is very difficult to distinguish the difference between the weld and the parent metal. Where you often will find differences will be when it comes to using these 'hollow' sections for pressurised pipework - as ERW ahs a lower 'joint' factor than CDS which reduces the overall pressure rating of the pipe - guess what I've been doing at work recently
Interestingly, I did some work comparing the cost of ERW and CDS for a job at work and found there was no clear difference between the two, for some stuff one was cheaper than the other and for others the reverse applied. One other thing to bear in mind when ordering is that the tube is typically supplied in 'random' lengths - these are between 5.7m and 6.1m - it is easy to get caught out by this and assume they are 6m lenghts - been there done that
Cheers
Dave