poly threads do hold up better. cotton wears out, gets weak and breaks alot. you can re=quilt it if needed. 30 wt is actually a pretty heavy thread; we mostly use 40 wt threads for piecing i've used 50 wt and 100 wt too. the smaller the number the 'thicker' the thread...so your 30 wt thread is almost twice as thick as my 50 wt. also, if you use cotton thread i 'think' it will hold up better if the item is washed in cold water on gentle..