Really a matter of opinion. If you main goal is to see Glaciers then go Glacier Bay (which is only at the beginning and end of the Alaska cruise season). For north and southbound to me it's a toss of the coin. Some say the scenery builds as you go north - which is true the scenery is more dramatic farther north. We choose Southbound because we wanted to spend a few days in Alaska and since I'm one of these people that swears by flying in at least the day before - we opted to do Anchorage and Seward first - followed by the southbound cruise.
You will see glaciers along the way - I'm "told" not as dramatic as Glacier Bay - but you will get to visit Sitka - very beautiful and not part of the Glacier Bay. Also - there is always a risk that the bay will be too foggy and you won't even get in there (this is what I have read while researching my cruise).
Good luck in your decision and whatever you choose you won't regret it.
Hope 2 Sea U Onboard - Cheryl
Fantasy, Inspiration (2), Conquest (2), Spirit, Ecstasy (2)
Coming up: Triumph (November 28, 2009 - Penthouse - Sweet!)