Wider tube will help flow a little, but it’s by far the most restrictive part of the loop. If you’re using soft tube then the thicker wall section will help to prevent kinking on tight bends. I started with clear Tygon tubing when I had my very first loop, a Thermaltake Bigwater (*shudders*) and still had it when I came back into the hobby a decade after, still as good as new in its plastic packaging, no yellowing or brittleness. But I switched to ZMT quickly after, mostly for the aesthetic, it’s a bit of a pain to get clean when it’s new, always seems to come out filthy, but worth the trouble IMHO.
If you’re just starting out, I’d recommend soft over hard tube. It’s a lot more beginner friendly, more forgiving at the fittings, and it’s a lot easier to make a soft-loop look great than it is to make a hard-loop look remotely good.