My son and daughter were both anterior and this baby is posterior and WOW, the difference!!! I have been able to feel this baby like forever, I am now 28 weeks and can feel everything, every tiny movement of his hands etc. I can see him as well! With my other 2, anterior placenta pregnancies it was not nearly like that! I think the difference is amazing and I have heard others that experienced the same thing! I am loving it ;)
Picking up a heartbeat though shouldnt make a difference, but at 17 weeks its very early to pick up a heartbeat anyway. Over here they start at around 26 weeks to check heartbeat from "outside", before they scan to check on it.