Experts (experts at what, precisely?) will tell you it must be just so,
but as an expert on my boat, I have no idea what woud suit you and your
useage.

I have the impression that a whisker pole is slighter than a spinnaker
pole, and there are experts who will tell you all about that.

For me, a pole is a pole. It should be just the right length to do
what you want it to do. My boat came with a spinnaker pole, replete
with bridle and spinnaker snaps on each end well suited for it's
official purpose, but with no spinnaker. So, I used it whenever I
needed a spar for something. Eventually, I realized that if siezed by a
link of chain at the stem head and raised from the deck enough, it
would pass in front of the lower foreward shrouds, and that If I cut up
an old working jib, it could serve as a boom for a seriously flat,
close in sheeted self tacking jib, my greatest joy, sailing in a river
channel.

It was also good as a whisker pole if the bigger jib was poled out
using it snapped on to a stanction base instead of the mast, as it was
a bit short otherwise. I do not want to have to stow several different
extra spars.

Sailors improvise, and fiddle with stuff on hand until they believe
they have an improvement in their rig, it's all a part of messing about
in boats.

A spar used to hold a jib outboard for dead down wind use need not
stretch the sail out all the way, unless you are racing for a new tin
cup, needed to beg for money to get a spinnaker and do it right, but is
needed to help the sail stay full when buffetted by turbulence typical
near the wind shadow of the main, if you want to fly the main.

Only you know all about your sail inventory, rig and cruising and
racing requirements, and aggressiveness.

Only you can decide if you want a serious heavy weather spinnaker
racing strut, or if a smaller, lighter, adjustable, more bendy spar
would suit you better. Maybe you "need" both?

A long, skinny pole with a huge sail will bend and break in a gust.

Your sailmaker would advise you best.

Terry K