On first thoughts, since you have already developed and deployed your applications, SoftLayer would be cheaper.
But you can subscribe to BlueMix services for very short time intervals, so you will not have to pay for a full years software licence fees; just pay for the short intervals, Whereas on Softlayer , you may have paid up for a full years licence fee for the various softwares required to run your applications.
So you will have to factor in these and decide what is economical for you.