I agree, Vacationstogo.com has the best search functions. Chose your month(s) and region. Then I would sort by star rating. Generally, the more luxurious, the smaller the ships. And in that sort, vtg sorts by *, and date in each * group. For example, it will sort all the 6* by date, then the 5.5 *, then the 5*.

My personal fave is Oceania (larger ships, they have 2 sizes). Mainly because we don't drink and do our own shore excursions, so we're not paying for those things we don't partake in. Azamara is nice, but the cabins are tiny. We've just done Seabourn South America coastal Lima to Santiago.

Mostly, cruise ships follow a certain route pattern. Summers in Europe, cross the Atlantic in the fall, either head down the coasts of South America fall/ winter to do Antarctica, or start their Caribbean season in the fall. Then TATL back across in the spring to Europe. Some do Asia, and head to Alaska spring/summer. Some go thru Panama Canal and head to Alaska. April is the "end" of Caribbean sailing season for a lot of ships.