Lefty nailed it. Properly made, there's little if any difference in performance. Look at some of the bows that have been tested to be the best performers on the market today--Adcock/A&H--why would they go to the trouble of all that work to make a better performer, then put a string on their bows that would slow it down?
Over the years I've had folks tell me this material was quieter, that one, etc. In my experience, tuning has more to do with it than string material. Most folks I've heard from have said a flemish string is noticeably quieter than an endless, but obviously there are exceptions.
Along with how the string is made, I've run across a few bows that just seemed to "like" one type string or material better than the other.
Personally, I don't care to ever shoot a dacron string again. I don't like dealing with the stretch, creep, and extra hand shock compared to the low-stretch materials like Dynaflight '97 and 8125. I'd still use them on my personal bows even if they were a little slower than dacron.
Speaking from personal experience, I get comments on a regular basis about how quiet my bows shoot. Tune the bow without any silencers to find the "sweet spot" (where you have the least noise and vibration), then after you put the silencers on, tune the silencers. Moving them up or down the string as little as an inch can make a big difference on some bows. I use one set of cat whiskers--might be able to use less, I've just never tried it. I get plenty of speed and a very quiet shot, so I'm not going to try to fix what's not broken.
I make my own, I can make either endless or flemish, and I have 4 different types of string material on hand--so it's not like I'm trying to be biased for one or the other. I just use what works best for me.
Matabele, the only way you'll know for sure which you prefer is to try them both and see. My bet is you won't find much if any difference in performance, and usually there will be more noise from an endless than flemish before you take any steps to silence them, especially if the loops are padded on the flemish.
Chad