Unfortunately for me I am married to a school teacher (who still has a few more years to work) so therefor we can really only travel at a few select times of year. This limits us in a few ways as well as impacts the budget. I would be very interested in a North America to Europe or Australia cruise but they go while school is in session. We are doing something out of the ordinary in we are doing a European Cruise in May but this took 2 years of planning to get the leave from work et all.
As for cruise lines. Celebrity is our favourite but is not always budget friendly. Our past cruises have been on Celebrity, Carnival and Royal. The May one is also on Royal. For budget I enjoy MSC as well. As noted previously it is destinations that make the big decisions for us mostly. I generally try to get at least 1 new place on each cruise, must admit this is getting tougher now that I have been to about 50 different ports. There are still a few places left so we can probably do once a year until my wife retires from teaching and then try a few of the off peak times and places.