HG/SS by a landslide. A majority of the Sinnoh Pokemon are huge disappointments to me, and I found the region in general annoying to traverse. Graphics were quite nasty, too. I'm replaying Diamond version right now, and oh god, I cannot get over how bad some of it looks.
However, Johto is probably my second favourite region next to Hoenn, and although a lot of the Pokemon introduced were somewhat lame, the good ones far outnumbered the bad ones in my opinion. Plus, so many new features. The running shoes button needs to be brought back some day, as does the Pokemon following you feature. So HG/SS wins, basically.