It depends...
For me there's 2 different types of long series, and I classify them as such:
There's the "InuYasha" long series. This is the one in which there are a lot of fillers and nothing much gets done, and at the end of the series you kind of wonder why you've watched 100+ episodes of this.
Then there are series like Bleach or Dragonball, which aren't as bad to muddle through.
I find that most series under 50 episodes are usually quite good, and I usually like a manga no matter how long it is. Money isn't a problem when it comes to manga for there is the library network! -ish a dork-
Anyway, for the most part I prefer shorter series; less chance of filler.
One of my favorite mangas that was serialized in "Shojo Beat" is "The Yume Kira Dream Shoppe", and that was only one normally sized volume long...