Well since you already know a bit of java i think it would take you roughly, two months to prepare for the 1.5 exam. Thats just what I think anyway. Which one you want to go for is up to you. Search for topics here that compare these certs.
Hi, Well its always better to give latest exam. But still it's your choice. First of all there are some additional topics in 1.5. And its importent on which verison you are currently working, if you are working on 1.4, then its better to give 1.4 exam. About preparation time.... its depends how much time you give everyday. If you have free time you can study entire day, but if you are working, then after job only you can work. so its depends on how much time you spend on preparation than how much days.
One more thing, spend more time on mock exams to score better.
I would say, that depends on how comfortable you are with the syllabus of that particular version. I would suggest you to go through the syllabus once and know what are the differences between the two exam versions and then decide for your self.