The detectives in the two stories play a very different role.
Sherlock Holmes is the main character and he is inn all of the scenes in the play where as Lieutenant Powers only is in a few scenes at the end. The whole of “the speckled band” is Holmes prying around finding out clues and information where as Powers only arrives at the end of the story and solves the case by tricking Ardis with brute force and underhanded means, whereas Holmes uses his witsHolmes is an old fashioned detective but he is probably the most famous detective of all times. He has acquired this reputation by solving many exceptionally difficult cases, where as Lieutenant Powers seems to be a regular detective who has been trained in many techniques to solve the case.
In my opinion the two stories both have many similarities and many differences. This is because that they are two completely different styles of writing detectiveStories. The Speckled Band is the traditional style and Forever After is the American Hard boiled style. This is where nearly all the differences become noticed. Also the fact Forever After was written 70 years after The Speckled Band also creates lots of differences. They are also written in different tenses. The Speckled Band is being told as a story whereas Forever After is happening as it is told.