I've been eyeing them up for awhile now actually, but from previous experience from other pet sites I really need to ask this:

Are they even worth it? Some pet sites I've been on, their cash shop weapons (With as expensive as a lot of them get) were pretty bad compared to non cash shop weapons.

Are they worth it and if so, which one is the best? I want to go off on a limb and say Eternal because of its price, but you never know.

Nah, not really. I don't own any of the cash shop weapons myself. The only one that's kind of worth it is Rainbow Vortex. It's a once-per-battle full block with a small chance to freeze (I think, don't quote me on that). But honestly, there's a cheaper alternative in the Shinwa's Temple Shard, though you lose the freeze chance with it.

There are far better options you could invest in imo.

Confirming it does have a chance to freeze. I like the Rainbow Vortex. It's cool, but doesn't become useful til Tier 5.

Hold off on it and decide later once you get more into battling and your pet is further along. It's something you can live without, but does make some battles easier later on. An extra turn may or may not make a difference to you.

The rainbow vortex I would say is the only one really worth it if you can afford the investment. It has a medium chance to freeze so the freeze rate is good and it is also a OPB full blocker. It's nice since you could use it with a blaster to blast an enemy weapon, or use it to protect yourself and heal for a turn. Is it essential for battling? No. Does it make battling easier? Yes.

The sentinel is nice if you have legendary combo weapons as it boosts their stats and makes them better. As someone who battles with legendaries with a T7 pet, the heal the sentinel can add to your legendaries is nice, but not needed. I find it more of a wasted slot that could have went to another weapon, healer or blocker. Other than that, nothing else is really worth it unless you consider the weapons that cash shop challengers drop. You can't buy the weapons directly from the cash shop, but you can buy the opponent that drops the weapons as loot from the cash shop. Some of those are quite useful, but some are also cheap in user shops.

Even though Rainbow Vortex is one of the best weapons at what it does, the effect is often just not that powerful to begin with.

I leave it in my set, but it rarely seems to impact a battle.

The Shinwas Heart Pendant is nice, but definately not essential. It's an ok weapon with a heal and it scales through a couple of tiers. It's nice if you have multiple small pets to battle with, and you're too lazy to make different sets for all of them =D