Infinity is a good choice. I'd also take a look at Holland America ships. Holland America has been sailing in Alaska for 60+ years and they do a great job, too. Your best bet may be taking the information you've gotten on the internet and consulting with a travel agent who specializes in cruises, who can help you sort out the information and help you decide which cruise line, ship on that line, and itinerary best fit you.
May weather will be cool/cold and possibly rainy in Juneau and other places, but it's a gorgeous time of year. Take rain gear, as well as clothing you can layer under it, so that you can enjoy the scenery on the ship and in port without being uncomfortable. Especially in Glacier Bay, you'll want to be standing outside on the ship, watching the glaciers "calve" and it can be cold near those glaciers, even in the middle of summer.