I think your question is too specific. I atleast cannot for sure predict the outcome of that particular reaction. The Wittig reaction is a great reaction but can be limited in its use. The addition of a methylene group is how most often the Wittig is employed. Most will use Horner-Wadsworth for strictly E conversion. In your case I would look at Schlosser modification. This maybe a better way for you to convert to the diene you would like.
Also, you would most likely do a Witting using the conjugated aldehyde.
Yes, there may be few better ways to obtain that kind of dienes.
Maybe I have not made myself clear enough, I'm not trying to yield a certain compound, I'm trying to predict the result of that reaction.
The whole problems is:
First Ph
3P reacts with 3-bromoprop-1-ene, and then deprotonation with n-BuLi to get the ylide.
Finally we make it react with propanal to yield an alkene.
I'm expected to explain stereochemistry of the final product, and even when I know why stabilized ylides yield mostrly E product and unstabilized ones Z product, can't tell if this yilide, being not as stable as, for example, an α-carbonyl ylide, but way more stable than a simple alkane-ylide, would yield mostly E or if it would be something like E:Z = 60/40, or maybe mostly Z...
Maybe it's still a very specific question, in that case, I guess an explanation like this would work.
Wish I had acces to a lab to try it out
Running some experiments would make it clear.