June is a *splendid* time to go to Alaska! Our first Alaskan cruise was right around June 21, summer solstice, so there were very long days. We flew to Fairbanks (and did a 5 day land tour first), arriving at 3:30 in the morning, and it was still light outside!
We had rain only one day, and only for an hour or so. Others claim it "rains all the time in Ketichkan", but the day we were in port it was actually hot and sunny.
I understand that May and September (when we're next going, in 40 days) are the riskiest for bad weather -- cold, rain... June/July/August are considered "peak".
I think the one-way cruise, Seward - Vancouver is really nice -- you get to see a bit more of Alaska, and certainly more glaciers (when you're going I guess Princess' home port will be Whittier).
Since this is such a festive occasion for you, you might also want to consider spending a few days in Vancouver at the end of your trip --it is a lovely city, lots to see and do. Very cosmopolitan, yet the Canadian exchange rate is such that it's very favorable to Americans (you can get some very fine meals and lodging for significantly less than in the States).
I've only had experience on the Princess line -- which was wonderful. And which we're using again (Island Princess), so can't offer any comparisons. There is a Princess board here, too -- and you've probably read the reviews, which should help you decide.
Whichever line you choose -- Alaska is awesome, and I think you'll have a wonderful time!
