Please excuse the terrible photos but:
Here is Mondo slayer next to Sideshow clone for height comparison
Then next to Threezero Ultraman Taro
So pretty normal height for average 1:6 dude, however, next to the 3A/Threezero one things looks quite inconsistent
Need to find info on doomguy/slayers height to work out which one is wrong, either Mondo is underscaled or 3A is overscaled. Can't both be correct. Unless we are to assume his size can alter somehow due to mark of the slayer or the height of in game models changed between games... well in my mind they will have to be separate slayers (Doom is a multiverse story after all so I guess there can be multiple variants of the Doomguy)
Size difference extends to chainsaws and other accessories (Mondo on left, 3A on right)
(Please note I accidentally included the Crysis Nanosuits Typhoon rifle in pic, I was well past bed time already so picked it up in error, I edited the pic to avoid confusion, consider it included for size reference only)
The plasma rifle on the mondo one seems to have a small grip but when comparing to other 1:6 gun grips it looks ok, it is just smaller compared to the other included weapons such as the BFG. The grip will be puny in the 3A slayers large hands though.
3A/Threezero one wins in terms of paint and size (I always assumed the slayer was large but could be a misconception, will need to google to see for sure) while Mondo wins in terms of materials (no rubber), articulation (still limited but a little better) and range of accessories (the exclusive has that Lost Soul, 4 pairs hands total, 2 inter hangeable wrist blade attachments, shoulder canon, chainsaw, crucible blade, BFG, assult rifle, super shotgun and plasma rifle vs 3A's 4 total pairs hands, double barrel shotgun, combat shotgun and plasma rifle (edit: and chainsaw. Forgot that)