Yes, the two weapon sets are pretty damn-near the same.
From the announcement:
Designed by Alfonso Giraldes, the Sons of Horus Legion Mk IV Head, Body and Shoulder Pad Upgrades are a great way to theme and customise your Sons of Horus power armoured infantry.
The Mk II and Mk III power weapon sets, designed by Will Hayes contain a huge variety of weapons and equipment. The kits comprise of ten melta-bombs, five pairs of arms, three Phobos-pattern bolt pistols, three power fists, two lightning claws, two power axes, two power swords, two power gladius, one sheathed gladius, two chainswords, two hand-flamers, two plasma pistols, two Volkite Serpentas, a thunder hammer, a power maul and a heavy chainsword. This cornucopia of wargear is perfect for use with our range of resin Space Marine Infantry, and is also fully compatible with Games Workshop’s plastic kits.
These models are available to pre-order now and will be despatched from Monday 30th September.