not sure why, it is actually a bit cheaper here on gOg thenover on Steam. For example, Worms Revolutions Gold Edition base price here is £17.09, but on Steam it is £18.99

I'm glad to finally see Worms Ultimate Mayhem here on GOG. One of my favourites from the many Worms games. My favourite one being Worms World Party, which I already have (I just wish the original was included with the remaster but oh well).
Anyway, I'll most likely get Worms Ultimate Mayhem eventually, preferebly when it'll be a bit cheaper (time's are difficult ._.), and I'm also thinking of getting Worms Forts Under Siege, is that any good?

Could have fooled me that it played differently. It's been so long since I last played it, any gameplay changes for the story missions in WUM aren't noticeable at all to me. *shrug*

I'm glad to see these games here. I'm a big fan of Worms!

The story missions are visually mostly the same. It's everything else that is different.

Weapon balance/behavior/looks, worm physics/handling, worm animations (death, idle etc., all are from Worms 4 only), terrain destruction, different music and sound effects (the Worms 3D menu song is still stuck in my head: "Shake your coconuts" from Junior Senior), some weapons from Worms 3D straight up missing (Uzi, Scales of Justice, Nuclear Test, Homing Pigeon, Blowpipe etc.) Or even extremely minor things, like worms floating and exploding when falling into water instead of just drowning. The AI in the missions also seemed to be behaving very differently.

I've played Worms 3D pretty much to death many years ago, so noticed all the differences very quickly. And in the vast majority of cases, a given thing has been replaced by the Worms 4 equivalent with no regard for the original.

It's missing? (cries)

Yep. It just uses the Worms 4 menu.

One more thing I forgot to mention, the Worms 3D random map generator is completely replaced by the one from Worms 4.

I guess I'll be keeping that retail copy of Worms 3D then.