Yes the op3 is an upgrade from a s5. There is also the honor 8 for £350 from 3. Apparantly it comes unlocked straight from Huawei's factory but if that's wrong you would have to get an unlock code from ebay for a few quid. What things are important to you in a phone? The s6 has pretty bad battery life from what I've heard.
Op3 pros: very powerful, lots of ram for multitasking, metal body, near stock android and so the phone will feel snappier then some heavier skinned versions, good ROM support if you're into rooting, loud headphone jack , fast finger scanner. The biggest con is the screen, Full HD and pentile on a 5.5" screen means it isn't the sharpest around.
The honor 8 pros: Better camera, sharper screen, battery should be a little better than the op3's, powerful cpu, fast finger scanner. Cons: Less powerful graphics so gaming is worse than on the op3 and the honor 8 is heavily skinned i.e. it isn't close to stock android but it's smooth with nova launcher installed (I have the Huawei p9 that's virtually the same phone apart from aesthetics), headphone jack isn't as loud as on the op3's so for driving large headphones the honor 8 isn't as good.
Grey areas: 5.5" screen on the op3, 5.2" screen on the honor 8. The op3 has 6gb's of ram but it hasn't been clear if it has better multi-tasking or not than the honor, with 4GB's, because one plus had concerns over battery life, so they limited its use when it was first launched but mb that has changed with an update? Speaker quality idk about that.
I think the op3 and honor 8 are the best new phones for the money out right now.
One big reason to go with the op3 over the honor 8 is the emui skin that's on the honor. It can be fiddly to use and annoying. It does have some extra features like a call blocker, it can automatically close the apps you select that run in the background to save battery, a firewall to stop apps that you don't trust from accessing the internet, but I would still prefer stock android . If the honor came with stock android I would reccommend it without any hesitation but IMO emui kinda sucks. Having said that, on my p9 I don't mind using it too much now I'm use to it and there is also a way to debloat it without having to root which I have used and it works. Both phones are more powerful than the Samsung S5.