Personally, I prefer the PAL ones (I swear this isn't European bias on my part lol). I like a lot of the colour choices they'd use for their boxes (Plus; they'd sometimes do their own custom designs like the Mario All-Stars box for example which I really like a lot). I understand how some might like the more uniform look the NA versions have though, but they do look very busy a lot of the time and that "Only for Nintendo" logo they have in the corner looks a bit tacky to me.
What's interesting is that later SNES releases ('95 onwards) in PAL regions started to kind of replicate the NA box art designs themselves. Take Terranigma for example:
These are the German, British & Irish (Top row), Spanish and Australian & New Zealand (Bottom row) releases. As you can see; subtle differences between them all even though they were trying to do the more uniform look the NA ones have, except for Australia and New Zealand who kept the classic PAL style box arts going (I think they got the best one out of this bunch if I'm being honest).