Basically,both these phase there are the same meanings.but if you would like to differentiate them,'先...然后...' first you can do,and then secondly you do another thing.the times order we can not transpose.must first and then secondly. and the '先。。。再。。。'means sometimes if you say a thing,about the timesheet,first for waht and then for what,it does not emphasis the order.at some time they are equally as well.

Their difference is just like the difference between "First....Next.." and "First...Then" in English. Not much different. In many cases, they are interchangeable.

it does have a minor difference: 先再 emphasizes 再 部分(notice it is fourth tone?), usually it is used when people did something wrong like they did b before a, then you say 不是这样， 你先a再b， 先然后combo emphasizes 先， used when you are giving instructions to someone. hope that helps! This is going to make you a native 中国人。： ）

没有大的区别。 first do something, then/and then/after that do something else 举个例子: 我先买菜，再洗菜，然后炒菜。 也可以是: 我先买菜，然后洗菜，再炒菜。

no significant difference. use any one you like.