Ultracam may be less durable, but unless your shooting many tens of thousands of shots before changing out strings, you will probably never experience it. It stretches less than any other string material, and does not fuzz as easily as the other materials. It makes a great string if you absolutely don't want it to stretch.
Pre-stretching a string will shorten the break-in period. When I build a string, I stretch it with about 250-300 lbs of pressure. It takes about 5-10 shots to set the string and from that point on, it won't creep or stretch even 1/8". If you don't pre-stretch, then expect a longer break-in period (maybe a couple hundred shots), before you can tune your set-up. If it's properly built, it won't creep. If it's not constructed in the right way, even the best of string materials, will continue to creep and cause ongoing problems with your bow's tune.