It depends on your boat. Only wire will give you freedom from stretch, but it is certainly possible to use rope but sail adjustment may suffer a bit. We used cord on our 22' Cirrus 30 years ago happily.
Replaced my kevlar/SS halyard 5 years ago.Dyneema is the way to go. Look on ebay its 1/2 the price than the chandlerys. You may need to change the mast head sheeve.
You should check the sheaves are capable of taking the diameter of rope you intend to use. Sheaves used for wire tend to be a tighter radius than those for rope.
David.
I've replaced my wire halyards with dyneema, and wouldn't go back. Braided polyester is ok, Marlowbraid being the best IMHO, but does show stretch. As said, you need to check the sheave groove profile. There is a rope called "Dyneema Cruising", recently marketed by English Braids (?), considerably cheaper than most other dyneema ropes, but I have no experience of it, might be a good option.
I still have wire/rope on my mainsail but went to all spectra rope some time back for the jib. The jib halyard certainly stretches and has to be retensioned after a few minutes of sailing. However I replaced it with a thicker rope which is a little better on the stretch. Certainly wire/rope is expensive to replace if you caan get someone to do the splice. olewill
I replaced the rome wire halyards on my boat last year. Needed to fit a new halyard box and sheaves on the mast. The old sheaves were grooves and would have damages the new rope halyard.
It was a complete pain as i had to replace the jib halyard box but i managed to get it sorted. i much prefer the rope only setup. It was impossible to replace with a mouse halyard. i can now.
Of the textile replacements for wire Dyneema/Spectra is the stronger, Vectran the least stretchy. Both these two are a pretty good replacement for wire if you increase diameter of the rope by 20% over the wire. An alternative, less stretchy and marginally stronger than any of the other textile ropes is Kevlar - this is unreliable, suffering from fatigue mode failure and being very prone to UV degradation.
Having used all three I would plump for Dyneema/Spectra even though it is very prone to torsional tangling. There are two types, both braid on braid, the cruising version (usually cheaper) has a softer outer then the racing version and is less likely to tangle, but if you're using rope clutches will wear out far sooner.
I have long since replaced all my halyards with Dyneema/Spectra and am gradually working through the control-lines. The major advantage is that you can safely reduce rope-size by at least one (ie 2mm) compared to prestretch and have far freer-running tackles.
You should have dual-pulleys, having a smaller groove in the centre for the wire and a larger for the textile tail. If you have well and good, if not you'll need to change the blocks. there is slightly more stretch than wire but it's irrelevant a the difference in stretch pales into insignificance compared to the stretch on the mainsail luff.
IMHO wire-rope halyards are obsolescent, increasing weight aloft, being more prone to jams and producing far more frictional resistance.
The luff on my main is marginally under 12m, so the advantages, of textile over wire, may be less with larger mains.