if i make good money this time , i'll ___all my debts first.

pay off 的一个意思是专指还清债务
而pay back指的是报答或付回什么代价

pay back
(1)
报复 (= pay off, pay out, serve out)
I'll certainly pay you back for what you did to me!
"你那样对待我,我一定要回敬你的!"
(2)
偿还；偿付
How can I pay you back for all your kindness?
你的这番好意我该怎么报答呢?

pay off
(1)
还清；偿清
(2)
报复；偿还
(3)
结清工资解雇（某人）
(4)
成功
Did your plan pay off?
你的计划成功了吗？
Our plan certainly paid off; it was a great idea.
"我们的计划当然成功了,那是个很好的主意。"

pay off
还清(债务等)付清, 报复, 赢利
pay back
偿还(借款), 报答, 报复

没什么大的区别