Is anything wrong with your 300MHz system? Maybe you could merely upgrade the cpu. You would be buying 2 very similar systems. Does your 300MHz celeron has a cache?

^Yeah, cache will make a big difference. If you have an original Celeron without L2 cache, that Pentium II will blow it out of the water. However, If it's a Celeron 300A, it'll blow that Pentium II out of the water. With Slot I CPUs, in general, clock speed isn't nearly as significant as cache configuration and bus speed.

Either way though, if you get a good deal on it (I wouldn't pay more than $30 but where I live, PCs from that vintage have little to no value), go for it. If nothing more, you have a cache of spare parts.

I'm actually giving the Celeron to my brother and I'm buying a new (retro) PC to use at my new place. I'm sorry, I don't know about cache, is there any way to find out?

You can run a CPU identification utility to give you an idea which Celeron CPU you have.
Alternatively, your can unplug the CPU from the motherboard and look up the serial number directly from the CPU.
However, the first option is much easier and less risky.

Thank you, I just fount out that it's a Celeron 300.

If you do buy the Pentium II, run some benchmark comparisons between the two.
You can then decide which one you want to keep.

Thank you all for the helpful advices. I think I will go for that P2 for now.

yea a cacheless Celeron 300 would loose in benchmarks to a PEntium 166 often 🤣.