You pays yer money
and takes your choice.
Braid on braid (a.k.a. double braid) is fine... its cheap
and reliable. The downside is it stretches and it is relatively low strength (compared to other exotics), so you end up using a larger diameter than if you went with exotics
Spectra is better than double braid - stronger and less stretchy, but also more expensive.
Vectran is better still, even stronger, even less stretchy, but even more expensive.
For a 40' boat you might be looking at, for a main halyard
, 9/16 or 1/2" double braid, 7/16" or 3/8" spectra, 3/8" or 5/16" vectran...
Shop around and you can find some pretty big variation in per foot price, so don't discount going for a "exotic", especially if you are comparing a thicker 1/2" double braid to a thinner 3/8 spectra...
Bottom line, the more exotic lines will give better sailing performance - you will be able to maintain better luff tension, and will also give you marginally less weight aloft. If sailing performance (particularly upwind) is important to you, then maybe you should consider spending a little more on your halyard. If cost is the major factor, double braid will do the job.