Art-wise, I prefer the new Pikachu, especially the redrawn version for FR/LG. That art just looks like Pikachu. Probably because I played FR/LG more than R/B/Y. But the newer Pikachu art just seems to have more personality in it. The old art is just sitting there, maybe slightly sparking at the cheeks. But the new art is happy, leaping about with glee apparent in its face.
Plush-wise, I prefer my old Pikachu plush compared to the newer one. The old one is just more cuddly, while the new one is far too detailed. (Also my new one has a squished-in face so he looks awkward.)