If you choose the carbon: make sure it slides easily in and out of the lower mast - even when wet. The first bunch of carbon masts had a fitting problem. Later cured when the next batches hit the market.
Fitting should be as easy as with the old alloy masts!
Their still might be an early specimen on the shelf at the dealer’s...
Like Torrid wrote, the composite 'upper' was never meant to be 'faster'. That would conflict with the Laser religion.
Current pricing in the USA: composite $650; aluminum $210. That ratio makes me stick with what I have...
I've been sailing with the composite top section for just over a year now. It does trim different as it is 20%+ stiffer than the aluminum top section! Mine will swell when it gets hot making the fit nice and tight in the lower section, (I like this).
Being a heavier sailor when it blows the stiffer top section makes the rig a little more powerful. For the first time I'm able to ease the sail out in puffs, (with plenty of vang on) and sail fast while maintaining height. The aluminum tope section would sail block to block ALL the time unless it was above a solid 20. With the new section I can ease the sheet in breeze over 15.
The biggest adjustment I learned was using a lot of vang in light air, (like 5knots) where my upwind vang trim is just about block to block. DW it likes more vang as well because the MKII sail has more aggressive pre-cut luff curve in the main which takes more vang tension for the spar to bend and match the curve.
It took me a year to figure out how to sail the new section with the MKII sail. I'm finally there now.