Codeshares can matter a little bit. The main difference can be in terms of earning for your Frequent Flyer Program (so all of us QF members will always book AA codes over CX codes as we get bonus miles). The only other thing is that a codeshare might affect your chances of an op-up which is not really relevant for you.

Also be aware that CX regional can be quite bad, IMHO JL will be much better.

And I think there is no JL F to SIN currently.