I would normally suggest a southbound, but if you have folks to hang with in Seward, then a northbound will be great. You should be able to get a better deal on a northbound.
I honestly believe that the Radiance class ships are the best of the best. They have everything going for them and are a true pleasure to cruise. The Infinity is also a great ship, so it would be a difficult decision. If there are any Diamond members on Royal Caribbean, then the choice becomes a lot easier. The Radiance class ships offer a Concierge Lounge (free boooze) while the Infinity does not.
I just did the Infinity in Alaska last year...here is a link to the review:
No matter what you decide, it is a win-win situation. It is like trying to choose deserts. It is going to be great no matter what you choose.
Have a great cruise!