I have 400 or so 300gr TSX.
From all reports the 270 is a better choice in the H&H and the Ruger.
Is there a difference between the 270gr and 300gr besides the extra shank length on the base end?
Monometals like speed for best performance, so is it just the added velocity of the 270gr that makes it the better choice? Or is there a hollow point construction difference between those two bullet weights?

Monometals are also designed for the most likely velocity range and usage. Driving them faster than that is a positive bi-product many agree with.
I have done load and prep work for hunters taking rifles to Africa and I loaded the 300gn X as it was back then, with the hunters all delighted in the results on cape buffalo.

Between the 270 and 300 grainer, you would need to shoot several and average out the results and even then it would still be an opinion based on the experiences and data. ie both would work well and the buff won't care.

That is the reason so many experienced guys here recommend you shoot both and pick the most accurate in your rifles as that puts the emphasis on you, the user, to place the shot.

When getting ready for Africa, I worked up loads for both 270gr @2800 and 300gr @2600 out of my H&H.

PH said to use the 300's, so I did with no issues. Worked great on Sable, Roan, Zebra, Hartebeest and Impala.

I would have been just as comfortable with the 270gr.