I split my stay often have done Negril first then Portland and also the other way around. It makes more sense to do Negril at the end because of the travel time back to the airport. I drive mostly but a cheap way could be Knutsford you would just need to plan your arrival time around their shuttle departure time. Portland is truly special and worth the hassle to get there total different vibe from Negril and very lush and just plain breath taking.