Price & Quality of Rope
Here''s the deal on high tech halyards, you can typically go two to three sizes smaller with high tech halyards and still have less stretch. As a result they get pretty less expensive than they might appear at first glance. For example, I bought a just under 100 foot halyard in Technora 900 last summer for $225 including the splice and two sacrificial cover splices. The proper diameter polyester was roughly $180 with the splices. For the price of a good meal I ended with a lower stretch halyard and less friction to overcome when I raise the sail. Given the long life of modern halyards, the extra cost is something less than $5 to 10 dollars a year and well worth it. Especially if you intend to race.
While a line like Stayset ''X'' works fine for most smaller cruising boats, conventional polyester is stretchy enough to be a little dangerous. In a gust the halyard loads increase rapidly and the halyards stretch accordingly. In doing so they can power up the sails quickly. This results in an increased weather helm and heeling.
High tech line is almost mandatory if you want to take advantage of higher tech mainsails and jibs. The real advantage of radially cut sails, in polyester or kevlar is a greatly reduced stretch during a gust. This means that far less trimming is needed to maintain maximum speed and minimum heeling. You really are not able to take advantage of the sail''s lower stretch characteristics if the halyards are stretching and allowing the sails to power up.
One thing that I strongly recommend on any halyard is an sacrificial extra cover. This is stitched over the halyard at high chafe areas such as a stoppers and at the mast sheeve. It allows the stoppers to get a better grip on the halyard and also reduces both fatigue and chafe so that halyard lasts much longer. For example Laser 28 halyards went 14 years.