When I was old enough to start seeing the differences in steam locomotives, the Hudson, in particular the 5000 series NYC Hudsons appealed to me more than the Pennsy K4. I've read many snippets about both locomotives. Some touting the K4s superiority over the Hudson, some the other way round.
I still think the NYC Hudson designers hit a home run, while the Pennsy K4 designers hit a triple.
All of that said, what characteristics of each loco made it better or not as good as the other ?
Seems there are two different ideas encompassed in your post:
A) Which locomotive appeals most to me.
B) Which design was mechanically superior.
I'll answer the second question first.
The Hudson, by virtue of of the fact that it came along 13 years after the first K4s, incorporates many features that make it superior to the K4s. Among these are the front end throttle, combustion chamber, larger firebox afforded by the 4 wheel trailing truck, booster engine, mechanical stokers as built, etc.
As for appeal, we're all different.
Me?
I'll take the beefy "modernized" K4s with the cast steel pilot. I like the belpaire firebox. It gives an engine that "big shoulders" appearance.
As an aside, I like the four de-streamlined J3a Hudsons (5447, 5450, 5451, 5453) with the flat-faced smokeboxes (sometimes erroneously referred to as the "Selkirk front end") over the clean lines of the J1 and j3a Hudsons as delivered. I'm not a fan of dainty looking steam locomotives.