I'm making a world where sub-FTL interstellar travel is a thing, and so is interstellar war, but for the purpose of this question we'll assume the scale to be of a single system.
Spaceships have evolved quite a bit since our current era, and they range in size from several meters (something like 2 times larger than your average air superiority fighter, having a space-worthy manned vessel can't get any smaller than this in my universe) to up to 20 kilometers in the shape of a scaled-up space submarine.
Weapons comes in varying shape and size (proportional to the ship that bear them) and can be split in 3 categories:
Missiles : Just your everyday payload carrying self propelled firestick, you could also call them torpedoes at this point, it doesn't really makes a difference as far a I know. They won't get bigger in size than an ICBM and the tech didn't evolve much aside from targeting and space-worthiness.
Lasers: They are mainly used as a countermeasure and/or to mess with enemy targeting systems at close range (and occasionally to blind the enemy commander trough the window as a prank).
Railguns: This is where R&D was the most successful, since those guns can get pretty big (remember the 20km ships?) and the biggest projectile to date is a whopping 100m long and can travel at 30Km/s. Picture throwing Saturn V at the enemy vessel). Please note that for smaller projectiles the speed can get up to 150km/s, but that is a very specialized gun and the average is less that 100km/s for a decent gun.
Concerning energy requirements, capacitors went a long way and are now extremely efficient, as well as cooling, even in space (firing a railgun more than once won't melt it for the first couple consecutive shots) and fusion is the go-to mean of energy production across the civilization. Dyson sphere project started in some systems, and the question of the access to the host star sparked the wars in the systems.
Now for the question: Is it of any use to strap a nuclear payload to the railgun projectile or does the impact of a plain old solid steel slug at several kilometers per second outperform such a projectile?
Bonus point: If it ever gets efficient, at what scale? (from the smallest tactical nuke to a Saturn V sized monstrosity of sheer destruction)
EDIT: My question differs from space-born ship-to-ship combat 150 years from now w fission/fusion tech as it is about the efficiency of a specific type of weapon regardless of the fact that it may not be the best kind of weapon. Although the two questions are very much linked, I do not think this is a duplicate.