Mount Gay Extra Old versus Appleton Estate Rare Blend.
I didn't mess about with any quasi-scientific method here, I just poured out a measure of each rum and sipped them alternately.
First up, they're both really nice. They're also quite similar, both very smooth, fairly dry, well-aged rums with plenty of woody spice. If you just gave me a shot of one or the other and asked me which it was, I don't know how reliably I could identify it. On the other hand, in a properly-controlled A/B/X test, I am confident I could identify the rum correctly most of the time. They're not identical. I think the Appleton is slightly sweeter and fruitier than the Mount Gay, with the latter being slightly drier and woodier.
As you can see, I've already spent quite a bit of time with the Mount Gay, whereas I've only just started on the Appleton Estate. I think I slightly prefer the MG, but that could just be because I'm more used to it.
At this point I'm going to hang on to the last of the Mount Gay until this bottle of Appleton also starts to run low. Then I'll try comparing the two again to see whether I still prefer the MG or whether my taste has shifted.
As an added note, the Mount Gay is a little bit cheaper at around £33 a bottle versus £37 for the Appleton. It's not a huge difference but it's enough that finding that I prefer the cheaper bottle is a good result.