I think 快点儿走吧is much more often used than the sentence you mentionned: 快一点儿走吧. And for 走得快一点儿吧， we say more often:走快点儿吧。
And for the difference between 快点儿走吧 and 走快点儿吧is the following:
快点儿走吧：can be used when you are still at home, and somebody wants you to go out quickly. So the meaning is to leave quickly
走快点儿吧：is used when you are walking, and someone thinks you are walking slowly, he urges you to go fast.