This is another subject that calls for opinions.
I think that both lines are good.
The Tamiya sets were originally added to the first-run of 1/700 IJN kits by Tamiya, Hasegawa, Fujimi & Aoshima and called "Leviathan upgrades", back in the late 1980's. They were CONSIDERABLY better than the weapons and accessories found in those original early-1970's vintage kits. As far as I know Tamiya first boxed them as Ordinance Sets (too bad they couldn't have spelled it Ordnance) just a couple of years ago, identical to the Leviathan upgrade sprues.
The Skywave/ Pitroad sets are a little more varied. Starting in 1986, I think, Skywave began with their blue box Krivak kits to put weapons and accessories on a generic sprue, including many more different types than this particular kit requires. They developed into sets for various nations and different periods and types of ships. They suggested that you could buy these sets and use them to outfit conversions or scratchbuilts.
As Dreadnought52 said, besides producing complete plastic kits under the Pitroad label, they also produced resin ships under the Skywave label, and these required the purchase and use of these weapons sets for completion.
Opinion: I think some of the Skywave parts run a bit large for straight replacement onto some of the Tamiya/Hasegawa/Fujimi/Aoshima ships. I think Skywave's 1/700 scale feels a little bit larger than theirs. Check fit first.
Opinion: I think there is more detail molded onto the surface of the Skywave weapons than onto the Tamiya parts. In general, I think that is good.
Right now I am working with some Japanese seaplanes from the two (skywave and Tamiya) sets. There's not much to choose between the two Petes I'm working on. Each is off a bit in one shape or another; each has panel lines (eew, gross, in 1/700!)
Yes, the weapons sets are included 'as is' in many of the kits. (And the Skywave sets also appear in some DML/ Dragon/ German Revell kits, because those kits were originally manufactured by Skywave. If you buy enough of these kits, then you're right, you don't ever need to buy a weapons/ accessory set. But if you're just starting out, they might give you the extra light AA or liferafts or something to outfit a ship just the way you want.
(And, since you were going to ask, Midship Models has similar weapons sets in their destroyer kits, and released individually. It is a bit of a mixed blessing, and worthy of a a separate discussion of merit compared to the new Skywave E-6 set) The Misdhip Models Cleveland class light cruiser kits are straight Skywave moldings, with regular Skywave weapons sets included. )
Good luck,
Rick Heinbaugh