From my limited knowledge of xm so far....here goes...
 
Price - these days a sideshow costs approx 500 usd....a xm costs around 650 usd (900 sgd) the sideshow prices stays the same after purchase due to large edition sizes...the xm prices sky rocket due to very low edition sizes...all xm customers win as they know their purchases will go up in price...95% of the time....exceptions being kraven and antman solely due to low popularity of the character..

Head sculpts: xm gives at least 2 or more...sideshow tends to give one or more depending on if you purchased an exclusive or not...xm doesn't have a concept like that....some times xm head sculpts has traces of asianess but since u get more than one, you can always find one to your preference out of the 2-4 that xm includes....

Poses: both are good....since of late all most all xm poses are brilliant because they show the pre-production in advance and incorporate feedback of collectors...

Mold and cast: xm weighs more...most between 17kg - 40kg. Xm engineering is better....they somehow find a way to hide the seam...

Paint and finishing: xm paint job is bullet proof! You cant see the seams...in fact i cant remember seeing any seams at all....

Packaging: how many times have we seen pics of broken sideshows upon delivery...i think i saw about 3 cases of the latest hulk vs wolverine....i am getting three and i am dead worried too!!! I have never seen a broken xm product....in fact u get spares for some items!!

Clothes: if u like mix media....xm doesn't....so sideshow wins...but take good care...

Accessibility: xm has limited purchases of only one per person of each character...their license allows them only to sell in east asia and europe...xm is hard to get

Franchises: xm has DC as well...this article is out dated.... they make B list characters as well...they make them so good that people who never dreamed of buying them pull the trigger! Eg: mysterio

Uniqueness: Xm spares no expenses when it comes to quality and finish....iron man mk7 comes with two bodies for two poses....you are getting two statues instead of one...lol....the bike statues have metal pieces and the cost is still around the same....Ghost Rider sold for SGD900 i think...Punisher also 850 or 900....

Future: xm has taken the statue world to a new level with their massive diorama series....and bike series....await for more surprises!