what size four blade prop do I need?

[bigeasy1]

I'm currently running a 15-1/2" x 17 pitch Mirage stainless prop on my 23 tsunami I/O with a 250hp Mercruiser and Alpha drive.
It does the job perfectly, and seems to be the best prop for my use.
However I want to get a spare prop and thought a four blade Aluminum would be a good choice, as Im told that they are better at low speed control like when docking, or backing.Im even thinking that maybe it would be a better full time prop and keeping the Mirage as a spare.
Several people have said that the extra weight of a stainless prop shortens the life of an outdrive.

So anyway what size four blade would put me in the ballpark.Any help would be appreciated